This Boy Suffers from Crystallization

High school boy Ayumu Tamari finds it hard to attend his classes especially since he had to repeat the year due to suffering from a strange syndrome which makes parts of its body's skin turn into a crystal whenever he experiences a strong emotion or stress. To the rescue comes his homeroom teacher Kouya Onihara, who also happens to love crystals. The relationship between them grows as Tamari tries to fit into his classes and other daily activities thanks to the pleasant time he spends in Onihara's office, which allows him to relax more (and crystallize less) acting more naturally towards his classmates.

Synopsis from Anime News Network