You will certainly need to have your high school diploma in order to enter a seminary and study directly for the priesthood. If you struggle in school you should keep at it, and offer up your efforts to God each day. Studies are necessary but not the most important part of becoming a priest. It is more important that you be a good person, honest, holy, hardworking, charitable, and especially that you love Christ very personally.

Normally you can enter a college seminary right after high school, but this depends on the diocese or religious order you want to join. So, if you want to be a priest in your home diocese speak to the vocation director to see what he recommends, or if you are interested in a religious order get in touch with them for the same reason.

God bless,