The main role of the teacher in a school laboratory is evident: They are there to educate students. It is the teacher's job to describe which theoretical topic students have to test in practice, which tools the students need and how to use them. In addition, teachers must explain each chemical reaction or the phenomena of a physics experiment, why those events happened, based on which laws and their importance. Students may have fun during a task in the school laboratory, but it still is a lesson.
Before each experiment, the teacher must inspect the laboratory thoroughly. The main concerns are the cleanliness of each tool and the laboratory in general, as well as the tools' functionality. Such issues can compromise the success of an experiment, but -- most importantly -- they may harm students. For example, teachers must spot broken test tubes that can cut students and check if electrical appliances are working properly. Spilled chemicals are also dangerous for students.
During the tests, teachers must always keep an eye to ensure everyone's safety. Reckless behavior by students, such as playing with laboratory tools, can be dangerous, and teachers must be firm to prevent it. In addition, students making noise may annoy other students who struggle to perform an experiment correctly. Teachers must also ensure all students follow the security procedures, such as washing their hands or wearing protective goggles.
After each experiment, teachers must instruct students how to clean the equipment they used and where to store them. In addition, the teacher has to keep a record of tools broken during the class, so that the school management can replace them. Leaving the school laboratory as you found it is highly important, as this way you can be sure nothing is lost under your supervision and also help the next teacher to use the lab.