Watch it first and decide.
I've been watching it with my 13 year old son, we've got to episode 11 and he's engrossed in it. So far most of the sexual stuff is implied and not that graphic. It's been more a few kissing and fondling shots, you are left in no doubt what happens but ... if they watch after 9pm shows it's pretty tame in many ways. To clarify, that's what I've seen so far, unless it's going to turn GoT on us?
Most of the story so far has been about how people do things and don't realise the cumulative part they play. Then the almost detective aspects of the story, you slowly see how the story lines come together. In many ways it's a fairly slow moving story and I think if a 13 year old isn't interested they will walk away in the first few episodes. It's not one of those where you are exposed to action or violence from the start.
My 13 year old has been covering many of the issues at secondary anyway, so I have no issue with the overall subject matter. Cut out the sexual aspects but still it has relevance to teens, kids and adults.
I agree with Rita that it's an 18 but aimed at a younger audience. When I first put it on my watch list it was a G, the same as Stranger Things, then it went to 18.
My 16 year old son binge watched it all and loved it. When he was younger he wasn't bullied as such but had a few things done to him.