They also school like that if there is something in the tank threatening them. You don't mention tankmates. What other fishes are with them?

Another ironic thing about aquaria, if fish have places to hide or shelter in, they will then often be "out front." What "structure" does your tank have in terms of decorations? Sometimes that is important to your guppies.

Also, what is the temperature of your tank? In the summer things can get hotter than usual. Water holds less oxygen than at cooler temperatures. Fish will huddle just under the water's surface because there is a little more oxygen there and they may desperately need more O2.

Related to that is how recently has you tank been established. Uncycled tanks may be oxygen poor. Also, such behavior is a good time for us (thee and me) to check on how recently and regularly we have been doing (often weekly) partial water changes. As nitrates gather in a tank, the fish can slowly, subtly, suffocate. :(

The hardest thing most aquarists face, is keeping up with partial water changes. None of the above may be the issue with your fish, But please give them some consideration.