See how things turn out for Term 2 and 3, do not base your decisions on JUST the first term.
I think you should also work on being more organised, because there are two reasons why 12 units isn't working out for a majority of people; the subjects are just not suited for them - and they chose subjects based on scaling or something on the same lines, meaning they might've never had any interest in it OR they are not being organised, leaving things to last minute - and placing in work that is not worth the grade they want. So if you're not enjoying art, sit down and just write down why - justify your thoughts, and have it written out so you clearly know why. Also, learn about what History Extension is like etc.
12 Units is a very secure situation, if you flopped in a 2U subject. 11 Units is the also secure because one bad unit can be replaced with a better one.
10 Units is preached in selective schools, because a majority of the student body has the confidence to do well and effectively invest all their time into those 10 units.