we say she tops the class every year meaning she has the greatest performance in her class per annum. Now how can we say someone has the weakest performance in a class?
There are several ways we can say someone was at the bottom of the class.
NOAD defines bottom as:
the lowest position in a competition or ranking
so that word can be used. The word you've used – weakest – can also be used, especially in conjunction with performer or performance.
Of all the students in my class last year, Bob was the weakest performer.
If you want a more humorous way to say it, you could say something like:
Bob was on the far left of my bell curve last year.
but expressions like that might be considered insensitive or unprofessional, and therefore should be used with caution.