I think it really happened, but exactly what could Joyce say - Honey, we don't want to have to send you back to the loony bin (the phrase used by a friend when I used of visit her in the bin.) It makes sense that she would express her concerns about school, and Buffy getting in trouble without really detailing all the trouble.

As for the comics - that's sort of a negative.They don't show parental concern at first but there is noting to show that they never had her hospitalized.