Hi Hel, I'm sorry to hear that you're feeling quite distressed at the moment.
It sounds like, reading your other posts, that you boyfriend has been responding quite well and in a supportive way to your discussions and communication, and also that the events you are describing (not responding to the text) have happened over a short period of time? You are also very aware of your feelings of anxiety around abandonment due to your past experiences with your ex, which is really great self-awareness, and it's great that you are going to see a counsellor to discuss and work through these.
Focusing on recent events, our brains can really quickly go into catastrophising mode when we're feeling anxious about a situation. This is because your brain wants to protect you from something bad happening, however can go into a bit of overdrive in its response to singular events.
You mention that your boyfriend's phone was taken away and so he wouldn't be able to respond (even if he was online before, I assume the phone was then taken away?), and that is causing you some stress. It is okay to give someone time to respond, but it can be stressful in the meantime, and that is where self soothing and self care activities can be useful to distract your brain from those thoughts by taking some time out for yourself(and a therapist/counselor can help you with the thought patterns themselves more). Here are some resources about that you might find useful:
This isn't to say that you should wait for him to initiate the next message, however it might be good to give him a bit of time to respond to your text (this is not days or anything, just not only 1 hour or so) before doing so. You mention he's also having some stressors at the moment, so this could look like checking in with him also in case his mind is preoccupied with his stressors. This isn't to say your needs also aren't valid, however it might show that you are also there to listen to him and acknowledge the things he has going on too.
You mention him not sitting next to you at school and also not texting you when he was online, and not being sure about whether he understands your needs. Would you be able to expand a bit more on what you are wanting with regards to when and how he is focusing his attention on you, rather than others, and whether and how you've communicated this?
When you say you "get paranoid" when he doesn't text you 1:1 when he's also messaging in group chats where you are present, what does what you want look like? When you say he should "know... (Or maybe he doesn’t know because he is a teenage boy and this is his first serious relationship.)" have you discussed this before or is it something you've thought he'd pick up on from previous discussions?