I don't know that anyone has mentioned this before and I'm sure it's a quick fix in digital form, but this spell specifically states in the description that it CHARMS those it effects, yet it is listed as an illusion spell, not an enchantment spell.  Could it be both?  Maybe, but something that charms is definitely an enchantement, n'est-ce pas?  I don't know why I haven't noticed it before, but I was going through options for an enchanter wizard a party I am running and, well...try to explain that one to a player.

You are incorrect, not all spells that apply the charmed condition are Enchantment spells. 

What is the determining factor?  We used to term it enchantment/charm (long time ago).  Even if it is an illusion spell (and always has been illusion/phantasm), should it not be included in the enchantment school, as well?  Just looking for a good explanation other than it has always been in that school.

None of the spells in D&D is classified under two schools, only ever one school. Since hypnotic pattern uses Illusion to impose the charmed condition, it is classified as an Illusion spell.

Thanks for the explanation.

Happy to help.