Macro to move data into a seperate worksheet in order

Hey everyone.

This is pretty much a last resort. For my A level coursework I need to create an automatic grading system. Now I've got the whole thing pretty much built but I'm stumped on one thing.

There are 12 units that need to be graded. Each student has their own student ID. There is a worksheet for portfolio units to be input and a worksheet for exam units.. I have a vlookup in both of those worksheets to allow the user to enter the student ID and unit number and it gets the information from a "lookuptable" worksheet.

There is a button at the bottom of both of those sheets saying "store and save"... I need something, anything that will transfer the data from some of those fields into a mastersheet - in order. But I also need it to check if there is already a record of that student ID in the worksheet and overwrite the information there instead of just creating a new line.

I've asked pretty much every person in my college. None of my tutors know how - It must be possible.. I just don't know where to start.. or even what the macro would be called!

Thanks for reading all of that, and I appreciate any and all help I can get!