The reason why it was so popular is because it was so relatable. You've obviously got all the lads trying to lose their virginity but you've also got relationship issues, parental seperation, bullying, banter between friends, trying to pass your driving test, the intimidating teachers, trying to pretend you're 18 and all the social awkwardness that happens at that age, when you don't really know what you're doing or where your life is going.
Obviously some of it is exaggerated for comedic value, but the first season n particular is a very accurate representation of what it's like being a 16 year old boy starting sixth form.
The characters are all very relatable as well. We all knew someone at college like Jay who exaggerated everything and obsessed about sex, we all knew a Neil who isn't the quickest of cats at the best of times, and we definitely all knew a Simon and a Will.
I must have seen every episode 20 times by now but I still find it funny.