In general, you can declare a UDT and make an array out if it. I don't know which version of VB you have, and I don't have access to any versions other than VB6 but hopefully it will help you out some.
Private Type Student
Grades(0 to 3) as Integer
Name as String
LetterGrade as String
' other parameters if needed
Dim students(0 to 4) as Student
As far as the menu goes, you'd have to check your version to see how to show/hide forms. I know newer versions vary wildly from what I'm familiar with.
For the user interface, an idea may be to use a listbox with each of the 5 students listed. Once the user clicks on a student, you can use that event to prompt for grades... the "index" within the listbox would give you the selected student.
To calculate the grades, loop through the students and each of their grades.
For i = 0 to 4 ' the students
sum = 0
For j = 0 to 3 ' the grades
sum = sum + students(i).Grades(j)
avg = sum / 4
If avg >= 90
students(i).LetterGrade = "A"
ElseIf avg >= 80
students(i).LetterGrade = "B"
Perhaps something along those lines. HTH