Unfortunately, Briggs & Stratton doesn't provide a class set of engines for free to Ag teachers anymore. Those days are behind us I'm afraid, but don't let your enthusiasm die there. When it comes down to it there are enough old lawnmowers in your community that you could collect enough for class. However, don't do the hunting yourself. Just spread the word to your students and parents of students. When they ask, "How can we help?". This is your answer. You can even make it extra credit for your students. The same goes for tool sets. Over time I'm sure your school will supply tool sets for use in the shop but in the mean time yard sales, estate sales, and donations are great places to get used tools. I know everyone wants new...I do too. That comes with time, but it's more important that their feet are in the shop learning a skill that will serve them as a home owner and/or in a career once they graduate high school.
There is a perfect time to start small engines...now. You'll never have enough engines, or tools, or some other necessity. Go with what you have. Only one engine? You talk and a student or two completes the step while everyone watches. Then onto the next set of students. Only one set of tools for four engines? The students shared glue, glitter, and scissors in elementary school so they have plenty of practice already. The key here is they must learn communication. They will have to plan out their shop time with the tools they can have access to. No, it's not ideal, but there are always lessons to learn beyond the mechanical skills. No matter what job they go into they'll have to learn to get along and prioritize their tasks. Your shop is a great place to solidify those interpersonal skills.