Average of best values in a series of values

Here is the problem:
I am a teacher using excel for my marks. My students are writing a short test at the start of every lesson...which amounts to 20 - 40 tests per term. I want to calculate the average of these tests. The nature of assessment is that a learner will perform good in some tests, bad in other.

I want to calculate the average for, say, only the best 15 marks of the total 25 tests that were written. Which combination of formulas will do that for me? All tests are counting ten marks, the average should then be a mark out of ten.

Thanks