Well, provided your teacher is doing their job, you will certainly learn all you need to know in order to pass your GCSE and A-level.
But they cannot teach you the entire history of music in four years! Even if you do a degree, you still won't learn everything. Learning is a lifetime process, not just something you do at school or university.
Hopefully, you will get a taste for it and wish to continue your own studies. You are luckier than I was; when I was doing my GCSEs there was no such thing as the Internet (yes - honest!!) so we didn't have such easy access to knowledge. We had to use books - you know, those rectangular things made out of paper and all stuck together! (sorry - just kidding!!!

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Anyway, your education will cover the basics, after that it is up to you to choose to study further into the aspects of music you enjoy the most. But there is always something new to learn. You learn till the day you die - that's the fun of it!