RollNo  Names    Class  Subject  Position
101     Anna     V      Maths    Average
102     Bob      V      Maths    Good
103     Charles  VI     Science  Good
104     Darwin   VI     Science  Improve
105     Eva      VII    English  Improve

$ sed -e 's/Good$/1/' -e 's/Average$/2/' -e 's/Improve$/3/' file.txt | sort -k5n | sed -e 's/1$/Good/' -e 's/2$/Average/' -e 's/3$/Improve/'
RollNo  Names    Class  Subject  Position
102     Bob      V      Maths    Good
103     Charles  VI     Science  Good
101     Anna     V      Maths    Average
104     Darwin   VI     Science  Improve
105     Eva      VII    English  Improve