That depends on a few other factors you don't mention: Is it a beginners course? What functions, operators or methods were teached directly before this homework? Would they be useful tools to solve the problem?

Generally the (mildly) difficult part seems to be to find a useful algorithm/mathematical formula to factor numbers, not the coding part as even your 16 line script will still fit into a single line if you just remove the newlines ;-)

PS: I just tried a very straightforward method and without trying to condense it used 91 characters with 8 statements, one of which was a while loop