School work is more important of course. Professional mangakas don't have school so they can produce 30+ pages per month, and don't forget, they have assistants while we don't

You can try drawing a bit when you have free time... for me I like to draw a bit before school, before club activities and during lunch time. You can never believe how many pages you can do in a month just with these times =D
Changing styles and storyline is you go on is normal. Everyone is improving on their drawing skills ^_^ And you have more new ideas as the story goes so it is not weird to sail away from the priginal storyline =D
-Ki