Are they school classrom type desks? Those I have seen usually have a plywood or composite top, with a laminate on the surface. If you want a nicer looking piece of furniture, you might try veneering the plywood. Lots of threads in this forum that discuss veneer. Ply or solid wood, it really comes down to how the kids treat it. If you have some that are likely to carve their initials in the top, I would avoid plywood, as it will chip and splinter. If you don't expect that sort of treatment, plywood is certainly more stable, and simpler to make panels from.