I this case, I think 我明天不能参加美术课 works too, if a bit awkward.
what about 可以 instead of 能?
Yes 可以 is correct
我明天不能参加美术课？ would this be okay? Could someone native or fluent confirm?
參加 is more like participate than attend
should be 我明天不能参加艺术课
I think “了” is missing at the end of this sentence. should be "我明天不能上美术课了。"