Assuming you do well it ends around May-June (I'm not entirely sure of the date,) you can fail the year and have to repeat (apparently if you fail your class fails too.)How long will it take to graduate and get the diploma from the school in the school start
If you want to skip the year you can buy a diploma from the shady guy near the industrial train station, but there is a lot of content at the school you would get cut off from.