One of the thorniest high school films you’re likely to see, “Luce” is about simmering racial tensions in a suburb and how fake smiles can cause them to boil over.

More fun, though, is that it’s a wrist-slap to liberal parenting.

Luce (Kelvin Harrison Jr., a major talent) is his class’ brightest star — an ace debater, skilled writer and an athlete. The perfect kid. He was adopted at age 9 from Eritrea by an average white couple, Amy (Naomi Watts) and Peter (Tim Roth), and the following 10 years were hunky-dory.

That is, until Luce turns in a paper written in the style and mindset of a 20th-century thinker: Frantz Fanon, a black philosopher who supported violent revolution. Luce’s teacher, Harriet (Octavia Spencer, intense as ever), is spooked.

But is this how the kid really feels? Or was he only fulfilling the assignment?

“Luce” is a taut, extremely watchable movie, though the dialogue could loosen up a touch. The film started out as a stage play by J.C. Lee, and it shows at times. The story’s most arresting thread is Luce’s adoptive parents, who are so concerned with molding him into a success in a prejudiced world, they forget that their son is a person.