OP, I used to work as a riding instructor in an approved (safe) BHS establishment. I've also worked training actors and on film sets.
This honestly all sounds really unsafe. If you've had two one hour lessons, it's totally unsafe for people to be cantering independently. There are situations where it can be safe/possible for people to canter with relatively little training (usually following another rider along a straight track or on the lunge) and it is (in some ways) easier than trot. In a school with other beginners is not a safe situation.
If I am teaching people to canter (usually between 6 weeks to 12 weeks in, sometimes longer, depending on their progress), I would always ensure the school is clear of other obstacles such as jumps, for the reasons you've described. Falling onto hard objects such as jump wings is a common cause of injury.
This all sounds really unsafe/dangerous to me, and personally, I wouldn't be going back. As someone else has said, it's not the instructor who will get hurt, it's you (and/or possibly the horse). Also, if you feel unsafe/scared, you will tense up and not be able to ride as effectively.
With all of that said, I do think rising trot is one of those things that some people just click with and others don't. I think you may have been unlucky to be in a group where it clicks quickly for everyone else- your experience is more normal ime. If you perhaps have less leg and core strength than others in the group, this may be why you're struggling more.
At home, you can practice by standing over a chair facing backwards then standing up and sitting down- try to do this without balancing on the chair. You don't need to fully straighten, when riding, just rise a few inches out of the saddle, rising too high is a common mistake. Also, ski-sits against a wall can help strengthen the right muscles.
If you want to ride more generally, and aren't just going along because it's a group thing, I'd definitely look for another stables. If you're under about 16st, there will be other options. Private lessons are good initially and different instructors do have different methods- maybe you just need to find the right person?