If your lab requires a large pot of boiling water, start the water for your students 15-20 minutes before class begins. Otherwise, they will spend half of the class period looking at the pot and waiting - that is, of course, assuming that they remember to start the water right away. It's not even necessary to fill the pots yourself; you could have the kids fill and cover them the day before during your lab prep.
Perhaps you are like I once was and you think to yourself "But I want for them to learn time management, how to fit together all the steps," blah, blah, blah. Screw it. Start the water, save yourselves the stress.
And while you're at it, preheat the ovens too. It's not like boiling water and preheating ovens are skills you have to continually review (yes, I know, you've got that one kitchen that still can't remember how to set up dish water, but you've just got to accept that and move on), and this will prevent a lot of stress and aggravation.
Exception: if you have block scheduling, I imagine you have time for the kids to do all this on their own. A couple of times a year I arrange for an "in-school" field trip where I have my Foods class for two consecutive class periods so we can actually cook a meal, eat, and clean all in one session - during these times, I don't start anything for them. But when you've got under 50 minutes for your labs, you've got find ways to save time and heartache.