I require that they first finish the lab as written and then go on as long as their experiment is relavent and safe. I encourage them to document and share their discoveries with the class. Ask Parents To Help Consider asking some parents to come in to help, either people you know or by sending home a general request for volunteers . Give them a copy of the experiment or the lab book and a copy of these instructions that emphasize the need to let the kids do the work. Parents are especially helpful in getting the materials ready beforehand and making sure everything gets repacked properly afterward.
Some teachers appreciate having extra adults help oversee the actual experiments.
One teacher would have one parent for each table of two teams if she could get the volunteers. Many teachers like having one extra adult to help keep an eye on things and clean up the inevitable messes. Some teachers prefer to go it alone. If the equipment is to be used by more than one teacher, I recommend at least having a parent to make sure the kit is kept in good order. It is very important to wrap up the experiment by sharing results, understanding differences, how what they saw relates to theory, interesting observations, etc.