NROTC Navy option mids find out about service assignment in early senior year (SEP/OCT). Service assignment is based upon GPA, NROTC performance (called aptitude), and the NROTC unit's CO's ranking/input. If a mid desires NSW/EOD, they must first get selected to attend a screener between their junior and senior year (this will serve as their 1/C cruise). You must apply for the screeners, and the NSW/EOD communities select who they will allow to attend; selection is based upon what I mentioned above, as well as their PRT/PST scores. The screeners give these two communities an opportunity to evaluate prospective future ensigns. In order to even be eligible for an NSW/EOD billet during service assignment, a mid has to have attended the respective screener.
A high GPA is great, strong PRT/PST scores are great, but there's more to it than that. Demonstrated leadership, teamwork, and MATURITY are just as important, if not more so. BTW, these communities take about 10 people each (from NROTC) most years out of over 850+ new ensigns that we produce.