Lena Headey says she won't be returning her son to L.A. anytime soon -- father's wishes be damned -- because the American education system sucks.
Headey's ex-husband, Peter Loughran, claims in new legal docs Lena agreed to return their 6-year-old son from the U.K. after she was done filming "Game of Thrones" ... but didn't.
Headey supposedly emailed Peter saying she was enrolling their son in a Yorkshire school because the kid was being held back by a subpar American school system.
Loughran insists the deadline for returning the kid was Sept. 5th, so she's brazenly violated their deal and his right to make decisions for his child. She begs to differ, saying he agreed the boy could live with her in England.
Loughran says he hasn't seen his kid in 8 weeks ... Headey hasn't even let them chat on the phone.
The former couple will have a showdown in an American court next month.