I have (somehow!) acquired a stable of vintage machines that I use to teach 4-Hers to sew. I've had no problem with featherweights, 99s or other old workhorses in the hands of beginning sewists. Boys really like the sewing machines - especially when you also insist they learn to clean and oil them. I, of course, am confident enough in my skills that I can keep them running from most mishaps. And I've also found that older machines are hard for them to muck up; whereas the newer ones can be pulled off time, subject to thread jams etc. Get the donated machines fixed and running and watch for good old (working) Singers and Kenmores and such. You might try to ensure all the machines take the same bobbins or at least have groups of them. And print off the manuals for reference; just do an online search and invest some money in downloadable pdfs. Just keep them simple. And remember to give the machines periodic breaks.
And yes I've let beginners use my machines. They are fancy computerized ones but so far, there have been no problems.