I really think mom is overreacting here. Usually, in these cases, it is the school personnel acting leftist, but it appears here that the school was enforcing an existing policy.
The students are expected to wear shirts with collars, and this particular shirt did not have a collar. I doubt the school was acting against support of the troops.
This was, apparently, a dress-down day:
“Student Cejai Taylor told Nashville Fox affiliate WZTV she wore the T-shirt right after her dad, Sgt. James Taylor, was sent on his sixth deployment overseas. Mahaffey requires all students to wear collared shirts, but has dress-down days once a month where they can wear jeans and school-approved shirts. The “Support Our Troops” shirt was not approved, and a teacher saw Taylor wearing it and told her to go to the office.”
Sew a collar on the shirt and send her back with it. Then we’ll see what the real problem is.