Smaller class sizes and more teachers. Part of why the problem kids are a problem is because the one teacher has to stop everything to deal with them.
Also, we need to stop pretending education is one size fits all. A lot of the “problem kids” are kids that aren’t well suited to sitting down for hours at a time having lessons dictated to them by an overworked, underpaid teacher. This is often especially true for young men. Our education system hasn’t been updated in over a century, really, in terms of this one size fits all approach. We need more outdoor-based education settings for these types of students, for example, leaving the classroom learning to those who are suited for it so they aren’t being distracted by the bored kids who need to be out doing stuff all day long and learning experientially.
The true figure is nowhere near 50%. These figures match other countries, as well. In general, only about 15-20% of the population consumes cannabis. The number is a bit higher if you include people who smoke like once a year.