A cheap rucksack is usually more than enough mate, after all you're going to school, which I assumed isn't that far away on a bicycle.
when I was a kids, my parents made me ride to school (which is 3 miles away), because at the time they were struggling, I have to use those really old canvas rucksack my father use when he was trekking Africa like this;
I'm sure you can just find a small lightweight rucksack, or cycle slower instead of being Eddy Merckx sweating from every orifice of your body.
also there's another option - saddlebag, you can get a good one easily for less than £50.
A cheap rucksack is usually more than enough mate, after all you're going to school, which I assumed isn't that far away on a bicycle.
when I was a kids, my parents made me ride to school (which is 3 miles away), because at the time they were struggling, I have to use those really old canvas rucksack my father use when he was trekking Africa like this;
I'm sure you can just find a small lightweight rucksack, or cycle slower instead of being Eddy Merckx sweating from every orifice of your body.
also there's another option - saddlebag, you can get a good one easily for less than £50.