create table Students
(
    FirstName nvarchar(50) not NULL,
    LastName nvarchar(50) not NULL,
    MiddleName nvarchar(50) not NULL,
    FullName as LastName + ' ' + left(FirstName, 1) + '.' + left(MiddleName, 1) + '.'
)

select FullName
from Students

FullName as ... persisted

create function dbo.tfFullName
(
    @firstName nvarchar(50),
    @lastName nvarchar(50),
    @middleName nvarchar(50)
)
returns table
as return
    select
        FullName = @lastName + ' ' + left(@firstName, 1) + '.' + left(@middleName, 1) + '.'
GO

select f.FullName
from Students s
    cross apply dbo.tfFullName(s.FirstName, s.LastName, s.MiddleName) f
GO