check with the embassy !
Best advice : check with your local US consular officer and ask, preferably in writing (e-mail), so that you have something on paper in case an immigration officer asks.
My personal experience with short accelerated programs is that US immigration requires either an M1 or a B1 or a B2 VISA. However, to get by on a B1/B2, it should be made credible that you 're not going to be longer in the US than about 2-3 weeks (at least not for training).
The more it sounds like a complete flight training programme, the more likely the need for an M1 is (to be obtained, at an extra cost and some hassle, through a SEVIS approved school sponsored application). A 10-day instrument rating without any previous training sounds ambitious.
The flight school obviously has a conflict of interest, and some of them are unethical in their advice. Being in the wrong immigration status is not a light matter. But for a short (finishing up ?) programme like that, it is not unlikely that all their European students came in on B1 or B2. Please note that the VISA waiver programme would not do the trick here (both the us govt website and correspondence with the US Embassy in Brussels confirm this).
Also don't forget the TSA category 3 application, which is mandatory for the instrument rating.