My DS is in second year of NJROTC. At least in his school, NJROTC is a one period a day class (Naval Science 1, Naval Science 2, etc.) for four years. In addition, there are after school NJROTC clubs (that are all optional), including: Drill (armed, unarmed, reg. and exposition), athletic comp., orienteering, rifle team and color guard. Also, there is community service and fund raising.
I think you would need to attend the one period a day classroom work in order to be in the program. I will ask the Naval Science Instructors about whether there is any way that a non-enrolled student can participate, but I kinda doubt it. As others have pointed out, there are other activities like Civil Air Patrol and Sea Cadets that are good too.