Weed is a good thing, and should be legalized to some degree, but it does have it's downsides. In the same way that too much cake will turn you fat and unhealthy, too much weed can do bad things to you.
The problem with weed is that it makes you feel good whilst doing nothing. If you feel good doing nothing for long enough, it takes a lot of effort and willpower to actually do something, like going to school every day. When you get to this point, you either go to school and hate it, or don't go at all, and get stoned instead. Either way, you don't learn very much, so you leave with fuck all to show for it, and limit your options in life.
There are other downsides to weed, but most of them are a result of prohibition, rather than the weed itself.
