When you google for images, you'll notice that as soon as you click a link, the top half of your screen will show a disclaimer telling you that the image might be copyright protected and that you can't just use it.
When you talk about school projects, you'll most likelly will not be distributing the things over the internet and thus not violate the copyright law. True, if you do the school assignment digitally on the pc, you're also violating that copyrightlaw, but your teachers must think: no way that this'll end up on the internet, and thus they don't fear in dealing with a lawsuit.
So to answer your question: Technically speaking, it depends on the site you're getting the image from, but usually its always a good idea to ask the author for permission, regardless if you're going to edit the image or not.