I'd never applied RHS and I'm not sure about previous years arrangement. But I bet the school should have experiences on arranging both N class and K class to submit application forms on the same day, and have sufficient knowledge on the traffic conditions nearby, especially during interviews / applications peak season. Hope the school will do better in the future.
Because SPN and SPK are 2 separates schools entertaining N1 and K1 separately. If you are applying SPK, all applicants are submitting form for K1.
The arguing point is, for those who applied RHS K1 is much less than those who applied N1. So, it would be more efficient to separate them into 2 groups of candidates.