Create a new version of paste:

local clientAssignmentDirectory = "usr/"
local userList = "user_list"
local adminUser = "KillaVanilla"
local adminPass = "something"
local lastAccessTimes = {}
local logfile = "server.log"
local logServ = 117

function log(stuff)
	local lHandle = fs.open(logfile, "a")
	lHandle.writeLine(stuff)
	lHandle.close()
end

local function spec_print(...)
	local args2 = {...}
	local str = ""
	for i,v in ipairs(args2) do
		str = str..v
	end
	rednet.send(logServ, textutils.serialize({"LOGSERV", "GradeServ", str}))
	if http then
		local wHandle = http.get("http://timeapi.org/utc/now")
		if wHandle then
			str = "[ RL TIME: "..wHandle.readAll().." UTC] "..str
		else
			str = "[ MC TIME: Day "..os.day().." @ "..textutils.formatTime(os.time()).."} "..str
		end
	end
	print(str)
	log(str)
end

local successMsg = textutils.serialize({"assignment", "return_code", "success"})

assert(fs.exists(userList), userList.." does not exist!")
for i,v in ipairs(rs.getSides()) do
	if peripheral.getType(v) == "modem" then
		rednet.open(v)
	end
end

if not fs.exists(clientAssignmentDirectory) then
	fs.makeDir(clientAssignmentDirectory)
end

local handle = io.open(userList, "r")
local cred_table = {}
for line in handle:lines() do
	string.gsub(line, "([%a%p%d]+)%s+([%a%p%d]+)", function(user, pass) cred_table[user] = pass if not fs.exists(fs.combine(clientAssignmentDirectory, user)) then fs.makeDir(fs.combine(clientAssignmentDirectory, user)) end end)
end
cred_table[adminUser] = adminPass
handle:close()

local function runTests(inputFile) -- run a simple pair of tests designed to catch (lazy) cheaters and weed out obviously-wrong answers.
	if not fs.exists(inputFile) then
		spec_print("Attempted to run test on nonexistent file: "..input)
		return "false", "File does not exist"
	end
	local iHandle = fs.open(inputFile, "r")
	local input = iHandle.readAll()
	iHandle.close()
	for i,v in ipairs(fs.list(clientAssignmentDirectory)) do
		if fs.isDir(fs.combine(clientAssignmentDirectory, v)) then
			for i,file in ipairs(fs.list(fs.combine(clientAssignmentDirectory, v))) do
				if file ~= "SanityTests.log" then
					local handle = fs.open(fs.combine(fs.combine(clientAssignmentDirectory, v), file), "r")
					if handle.readAll() == input then
						handle.close()
						return "false" -- cheater!
					end
					handle.close()
				end
			end
		end
	end
	local ok, err = pcall(loadstring(input))
	return tostring(ok)
end

local function getLoginCredent(id)
	for i,v in pairs(lastAccessTimes) do
		if v[2] == id then
			return true, i
		end
	end
end

local function runSanityTests()
	for userID,user in ipairs(fs.list(clientAssignmentDirectory)) do
		if fs.isDir(fs.combine(clientAssignmentDirectory, user)) then
			local sanityLog = fs.open(fs.combine(fs.combine(clientAssignmentDirectory, user), "SanityTests.log"), "w")
			for fileID, fileName in ipairs(fs.list(fs.combine(clientAssignmentDirectory, user))) do
				if fileName ~= "SanityTests.log" then
					sanityLog.writeLine(fileName..": "..runTests(fs.combine(fs.combine(clientAssignmentDirectory, user), fileName)))
				end
			end
			sanityLog.close()
		end
	end
end


local function listenerThread()
	while true do
		local id, msg = rednet.receive()
		msg = textutils.unserialize(msg)
		if msg then
			local hasLoggedIn, user = getLoginCredent(id)
			local isAdmin = false
			if hasLoggedIn then
				isAdmin = (user == adminUser)
			end
			if msg[1] == "assignment" then
				if msg[2] == "login" then
					if cred_table[msg[3]] == msg[4] then
						if msg[3] == adminUser then
							spec_print("ID "..id.." logged in as "..msg[3]..". (admin account)")
						else
							spec_print("ID "..id.." logged in as "..msg[3]..". (student account)")
						end
						lastAccessTimes[msg[3]] = {os.clock(), id}
						rednet.send(id, successMsg)
					end
				elseif hasLoggedIn then
					if msg[2] == "logout" then
						spec_print("ID "..id.." ("..user..") logged out.")
						lastAccessTimes[user] = nil
						rednet.send(id, successMsg)
					elseif msg[2] == "submit" then
						if not fs.exists(fs.combine(clientAssignmentDirectory..user, msg[3])) then
							local file = fs.open(fs.combine(clientAssignmentDirectory..user, msg[3]), "w")
							file.write(msg[4])
							file.close()
							spec_print("ID "..id.." ("..user..") submitted an assignment as file "..msg[3].." ("..fs.getSize(fs.combine(clientAssignmentDirectory..user, msg[3])).." bytes written).")
							rednet.send(id, successMsg)
						end
						lastAccessTimes[user][1] = os.clock()
					elseif msg[2] == "get_results" then
						spec_print("ID "..id.." ("..user..") downloaded his/her test results.")
						local file = fs.open(fs.combine(clientAssignmentDirectory..user, "SanityTests.log"), "r")
						rednet.send(id, textutils.serialize({"assignment", "results", file.readAll()}))
						file.close()
						lastAccessTimes[user][1] = os.clock()
					elseif isAdmin then
						local fullFileName = fs.combine(clientAssignmentDirectory, msg[3])
						if msg[2] == "get" then
							if fs.exists(fullFileName) then
								spec_print("ID "..id.." ("..user..") downloaded file "..fullFileName..".")
								local handle = fs.open(fullFileName, "r")
								rednet.send(id, textutils.serialize({"assignment", "file", msg[3], handle.readAll()}))
								handle.close()
							end
						elseif msg[2] == "list" then
							spec_print("ID "..id.." ("..user..") requested directory listing for "..fullFileName..".")
							if fs.exists(fullFileName) and fs.isDir(fullFileName) then
								rednet.send(id, textutils.serialize({"assignment", "list", msg[3], fs.list(fullFileName)}))
							end
						elseif msg[2] == "put" then
							local handle = fs.open(fullFileName, "w")
							handle.write(msg[4])
							handle.close()
							spec_print("ID "..id.." ("..user..") uploaded file "..fullFileName.." ("..fs.getSize(fullFileName).." bytes written).")
							rednet.send(id, successMsg)
						elseif msg[2] == "delete" then
							if fs.exists(fullfileName) then
								fs.delete(fullFileName)
								spec_print("ID "..id.." ("..user..") deleted file "..fullFileName..".")
							end
						end
					end
				end
			end
		end
	end
end

local function timedEventThread()
	local sanityTestItr = 0
	while true do
		local lastAccessTimes_clone = {}
		for i,v in pairs(lastAccessTimes) do
			if os.clock() < v[1]+300 then
				lastAccessTimes_clone[i] = v
			else
				spec_print("Automatically logging out ID "..v[2].." ("..i..") due to inactivity.")
			end
		end
		lastAccessTimes = lastAccessTimes_clone
		sanityTestItr = sanityTestItr+1
		if sanityTestItr >= 30 then
			runSanityTests()
			sanityTestItr = 0
		end
		os.sleep(1)
	end
end

spec_print("GradeServ V1.2 is ONLINE!")
parallel.waitForAll(listenerThread, timedEventThread)