The 4x objective is good for scanning a large number of cells. The 10x is good for general observations. As students move to the 40x remind them to Focus first at 10x. The cells will be white against a dark background. This is an excellent way to make observations.
Microscopes can get far out of alignment easily and quickly.
Students can end up spending an entire period looking at an air bubble or piece of dust if you do not check up on them. Using a video microscopy setup at the front of the class helps immensely in that it gives the students a starting point for comparison. Wipe up any sea water spilled with a cloth or paper towel dampened in tap water. Cover microscope with dust cover. Clean and dry slides and cover glasses.