I've had several candidates do this and it's definitely a good way to get into WP assuming as MemberLG mentioned, that you're aware of the consequences if that doesn't happen. We've been told several times that those 85 slots never are all filled - of course this could change. You need to be competitive on all fronts as well - academically qualified, leadership, sports, medical, CFA, etc. It's possible you could pass the METS physical (enlisted) and not pass the Dodmerb one for WP. It's also possible you could fail the CFA or get a 22 on one section of the ACT, etc. In any of those cases, you'd still be committed to the NG but would lose the shot at WP.
One more caveat is that you need to be 17 to enlist in the NG and they can't request your basic training date before then. If you don't get the basic training in over the summer between junior & senior year, even if you've already signed everything with the NG, you are not considered a "soldier" candidate in WP's eyes and would end up obligated to the guard if you didn't get in on your own. Depending on when your birthday is, this can be harder to juggle (obviously a May birthday is harder than a January one). Or the slot for basic training could just fall through or the unit might not be able to get you one - finding the right 10 week block that works with your schedule and the Army's during that short summer is definitely not a guarantee.
Overall though, if you are a competitive candidate, pass everything necessary, and complete that Basic Training, then yes, you pretty much are guaranteed a slot for WP. It's getting to that point that's the hard part and realizing how it'll affect your Plan B if it doesn't work out. I do know a lot of AROTC units though have a lot of cadets who are NG as well (Purdue is mostly that way) so college is still a possibility (as is becoming an officer) but just make sure you understand what you're committing to before you sign anything!