Open Clipart allows free use of their clipart. By free they state, We try to make it clear that you may use all clipart from Openclipart even for unlimited commercial use. You are free to use any of their clip art for commercial or non-commercial use, but do not necessarily have the right to modify it.
Open Clipart has a nice add on for Google Documents which allows you to search for clip art and drag and drop it into your document.
If the next time your students are doing a project, especially if they will be posting it online, you would like access to clipart "free of use restrictions", Open Clipart is a great candidate.