1) In lawsuit crazed world today, force them to sign a permission slip.

2) You should be more worried about them accidentally burning themselves than "lead" issues. This is less dangerous than arc welding in "shop class", which is common in numerous rural high schools in USA.

3) Before doing any soldering, teach them about safety issues of soldering and cleaning chemicals, give an unannounced pop quiz that doesn't count again them, summarize the test data for yourself, then tell them you will be giving another quiz, go over everything again and concentrate on their failures, then give a quiz that counts, use the results to make a decision what to do about the idjits that aren't paying attention to safety lessons.