Hey, I'm in my Gap Year studying Chemistry A-Level within the year sitting exams (a level) this cycle (started the course around 8 months ago). So maybe I can give you some insight. I'm doing AQA Chem, so keep that in mind.
Firstly, you're teacher is dead wrong and that's a terrible thing for them to say, and they are terrible at their job (an alarming amount of my teachers were also terrible during sixthform).
This specific method has worked wonders for me, that isn't to say it would work for you also.
To study content itself, what I've found to be the best method is to just watch Allery Chemistry's revision video on any topic (the ones with the black slides) I know they are called "revision" videos, but they are all you need to learn content. Then watch Elliot Rintoul if you don't understand something . Once you've watched the videos, go to PMT and do the topic questions for the topic you're learning. I've done this for every topic and it took me around 3 months doing 4 - 7 hours a day to learn the entire course (year 1 + 2), so i don't see why you cant do this for all of year 1. Looking back on Year 1, I honestly think you have enough time to get an A in your mock. This is the same method I did during my A-Levels in college, swapping out recourses for the relevant subjects, and got A* A* A A with <60% attendance. For Chemistry, I have also made flashcards on anki whilst watching mentioned videos. and completed them every day, purely because I need to learn lots of content fast. I believe this is the best way to do this.
Once you've done this spam past papers, this is probably the most important aspect of your studying.
To really fine tune your understanding, go to Chemrevise and use these notes to study reactions you must know. And a must for A-Level chemistry is to know the Organic Synthesis route map on there (although I don't think you need to do this for AS).
If UpLearn is working for you, that's great. But please don't use it if you don't see improvements. I personally tested it for AQA and thought it was quite all over the place and not very specific to the exam board. Especially compared to Allery Chemistry (and these are free and probably more concise). But if it works it works, everyone is different.
Also, outside of school, please balance your life. I was very into the gym during year 12 but stopped around 1/2 months before exams to study more. But I honestly feel like I would've benefited more if i reduced study time and didn't stop. I didn't repeat this mistake in year 13 and felt a massive difference in how I felt in general and had so much motivation. So please find time to do things outside of school and keep yourself healthy.
You can learn a lot of content quickly on your own if you actually try! Goodluck.