If the teacher writes the questions well enough, then cheating on the internet is extremely difficult <--- my opinion- I'm a teacher.
The main way anyone could cheat would be to come to forums (such as these) and ask their question. I've answered many homework questions before on here for students in math and physics... However, those questions have all been of the type where simply seeing an example is sufficient for many students to solve similar problems... kinda like "how do you factor x^2 - 4x -5" - showing them how to do a problem like that will hopefully help them to do the other problems on their homework. Teachers don't always provide good explanations or good examples. And, if the student is just being lazy and not trying to understand, then they're going to be screwed anyway when it's test time. I don't recall ever seeing a higher level thinking question being asked/answered on these forums (homework question). The types of questions are usually basic knowledge questions.
example: instead of asking: do dark colors absorb more heat energy from the sun than light colors?
A better question would be: If you lived in a hot climate, which of these colors would be suitable for painting your house: (list of colors) Why?