I share your sentiment very much brother, me and my friends went from:
- Boys only Duxi
- Followed by boys only boarding school
- Followed by boys only secondary school
When it was finally our choice, we of course, chose a mixed college, but by then we were the 4 mystic monks, who thought girls were unicorns that did not fart.
Of course Hooyo and Abo were suddenly: Well? Go swim!
Many of us learned to swim by just throwing ourselves into the deep end.
Its not as complicated as we often think it is. I remember us turning flirting into this complicated math equation, there were many popular flirting books about this back then, and we had them. I cant say they helped.
Believe it or not its not as bad as you think it is. You are 18 and are still in the tail end of things. if you have not, pick a suitable college, get involved in societies and in extracurricular activities. The after school events are what really helped me and my mates. If you are good at something lean into it, everybody wants someone who is useful in their team project or club.
All of this should give you ample opportunity to practice talking to girls, never mind flirting as it is hard to go from not being able to hold eye contact to flirting. It could be as inane as asking about yesterdays class or wanting to see their notes.
If you feel this is too big a gap for you, then on the early weekend days go to the shopping centres (malls) and just play dumb and talk to the girls (staff or not) there about some jeans, its quite and you dont know them.
The point is just focus on getting to a stage where you can hold a conversation with girls.
Also, dont neglect your appearances, first impressions are important and you will set yourself miles ahead of those that neglect it.
Two of those friends are married now and we occasionally laugh about it all. Good luck.