The other possibility is an interservice transfer. This was popular in the mid 80s (Marine to Navy after TBS) and early 90s (Navy to Marine after flight school getting sent to TBS). It just depends on who has openings. As to being guaranteed flight school... sort of. After the fall of the Soviet Union, many students were told you can leave or complete as an NFO, but we're not going to keep you as an aviator. Needs of the Navy and all that jazz.