Is it bad to force my students not to return early?
Actually returning early should be the norm. Return as soon as you can. There are at least two reasons for this, of which the first, efficiency, is the least important. But if you return early then needless statements won't be executed. Nor will you need to devise some special code to get to the end just so that you can return. 

However, the biggest and most important reason for returning early is that the programmer can put a limit on what she/he needs to retain mentally about the state of the computation. This is especially helpful in a sequential if situation. There is actually an Elementary Pattern about this ("Return, not else") that says that if you know what should be returned at the end of an if block, don't use an else, but instead end the block with the return. The reader/writer/updater then gets to ignore the previous state and start over in their thinking process. 

If you return only at the end, say "return result;" then you have to track all changes to result that might occur anywhere in the function/method. You have to understand it "as a whole" rather than incrementally. But the reading itself is normally sequential and incremental. 

Of course if you always write three statement methods, this won't occur, but few have the discipline to really do that. Sadly, some programmers can't understand their own code when it contains very many nested structures and tests. 

I'll also note that this is somewhat related to the idea that we generally try to avoid global variables in programming. Too much extraneous information can impinge on understanding. Being able to comprehend a program "one block at a time" without reference to others, even containing blocks, is a big advantage. 



Caveat. Of course, the code should be as clear as possible. If returning late makes it clearer, I wouldn't forbid it. But I think that is seldom the case. Forbidding early returns, however, is counterproductive.