Re: about training capacity
Is there a reason you don't tell the teenagers to slow down or do things with less force until they're able to do things safely? Clearly their skills or your ukemi (or probably both) aren't up to what they're trying to do. And it's a martial art -- lots of things can be dangerous. And since you're the one they're doing it to it's your right to say what's OK and what isn't.
In the long term if it was me I'd say learn more ukemi skills (e.g. one thing is how to move along with the way they're pulling/pushing you so as to minimize the forces on your body so you don't need to be as strong to 'take' the forces). But that takes time to learn and anyway no one is invincible. Especially if someone's doing things hard and clumsy -- bad combination... Lots of ways someone can seriously injure you. Sometimes you need to say 'no'.