As others have said, they were not classmates. But they had both served in the Mexican war. Grant resigned his commission but "went active" again after the CW started. Lee was very reluctant to go against the Union, but in the end his Loyalty to Virgina outweighed that to the Union. It was a different time, with most people identifying much more with their states than we do today.
All noted, thank you./Just Asking - seoul62......