Hi 刘则玮! Those is referring back to "the underlying merits": ...which are no less than the underlying merits of studying some core subjects. It seems to be phrased correctly to me.

I don’t know if it’s strictly speaking an incorrect sentence, but it’s a clumsy sentence and it makes it a little hard to read. I understand it to mean “Apart from that, the underlying merits of attending art classes are no less than the (underlying?) merits of studying some core subjects and will benefit the students in the long term.” I understand “underlying” to mean “real but not immediately obvious” in this context.