Update:
I took DS to school at 7 a.m. and walked him to the classroom. Another boy was ahead of us in the hallway, heading to the same classroom. When we arrived at the classroom, another student was already there. So that's three kids arriving at 7 a.m. to use the four available microscopes. A fourth kid showed up after we did.
The teacher saw DS and said, "Good morning." Then she saw me and said, "Can I help you?" I said, "Good morning. I'm (DS)'s mother, and I'm here to make sure he gets to do his experiment this morning." The teacher said, "Oh. Okay ..."
DS put his belongings down at a table with a microscope on it. He took a piece of paper out of his binder with the plan for his experiment and his data table. He approached the teacher and asked for salt. She said, "Salt? What kind of salt?" DS seemed confused. She then said, "Table salt?" DS said yes. She asked him why he wanted it. He explained that it was for the experiment. She asked to see his plan for the experiment. He got it from the table and read it to her. There was some back and forth between them about the nature of the experiment and DS's ideas about how it would work. She decided to give him the salt. This took about 10 minutes.
Getting out the salt involved opening a cabinet in the classroom, taking out a container, and giving DS a portion. He got the salt. The teacher had her back to me while she put some items back in the cabinet. I asked DS if he had what he needed. He said, yes, so I left without saying anything more.
Right now, his extra time before school has ended and he's now in his first period class. I'll have to wait until after school to find out whether he was able to do the whole experiment this morning.
I just want to say again how much I appreciate everyone's input on this. You guys are really great. Thank you.