Could space above a drop ceiling and below sound absorbing insulation increase echo in room?
The level of echo is based on the material of the suspended ceiling itself, not what's behind it. The sound insulation between the floor and ceiling will only reduce the sound travelling between the classroom and the level above. To cut down on the echo within the room, focus on sound absorbing material on the ceiling and walls of the classroom. Soft materials like fabric and carpet are best for this job.

Edit: To better answer your question, the only reason I can think of for laying the insulation is that you have no attachment points where the insulation would have less of an effect. Also, many builders put insulation between the joists, and the joists themselves transmit and possibly reflect some sound. But your concern about limiting access above the ceiling is valid. Also, many workers are unlikely to return the insulation to its proper location after moving it to do some work above the ceiling. Some light fixtures may generate excessive heat if insulation is laid directly on top. And finally, the added weight may not be good for the suspended ceiling, possibly causing the tiles to warp or other kinds of failure. For the ease of maintenance alone, I'd go for the insulation attached to the floor above.