Anarchis (spelling probably way off) is really easy to find and about impossible to kill, plus most fish won't eat it.
How about cory catfish for the bottom (instead of plecos)? They are a lively little bunch, and get along with angels. Cherry barbs add some activity and color, and aren't nippy. Livebearers would do well, although if it isn't well covered swordtails might jump. Angels would take care of any babies, so there wouldn't be overpopulation potential. A single female betta or dwarf gourami would also be interesting and add color. Depending on the size of the angels, a school of tetras or hatchets (if tank is covered) would be fine... if the angels are small enough, they won't go after them for lunch.
If it is an elementary school, I'd definitely go for cories and livebearers, because livebearers add lots of color. A dwarf gourami would be fine with them too, they are peaceful fish, and colorful (variety of colors to choose from).