nona the brit:
I come home from school at 3pm every day.
No, I don't use 24 hour clock. I don't know anyone who does use 24 hour clock in normal everyday life. You normally only find it used by the military or similar services, or on printed timetables (for example train timetables) to avoid any confusion.
If you do want to use it, then you can't use o'clock with the times. Its always 'hundred hours'. So, you would come home from school at 15 hundred hours (spoken) or 15.00 (written).