monitoring students

I am involved in education and have an interesting problem

Students are creating spreadsheets in the work that we do, simple example would be

place a value in column A and B, in C create the product of A and B, but hwat we are interested in is the formula the students use, ie =A1*B1. When they ahve put formula in we would like the sheet to indicate to us that the formula is correct. Problem nukmber of variations of acceptable formulae. Thought about data validation but this of course produces a drop down list for student to choose from. Tried Conditional formatting but to no avail.

any chance of any help from anyone. Please