Not so strange - just a child seeking out peers to have sex with. People assume that some magical maturity is conferred at age 18, but these days there just isn't all that much difference between these younger teachers and their students. They listen to the same music, watch the same TV shows, and have all of the same cultural references. There isn't the gulf between them that might have existed a few decades ago.
Schools might be wise to consider limiting the teaching of high school students to their staff members age 40 and older and let these younger teachers work with younger kids.
That’s a valid point, I’ve thought that this logic can be applied to police officers and their assignments as well. A 21 or 22 yr old officer shouldn’t be patrolling areas rife with college aged kids for similar reasons IMHO.