1) What is your 1st series pass rate for the CPL?
2) Ditto for the IR?
A little unfair this one, as few schools pick and choose their students, but at least it gives you a yardstick to compare with the opposition.
3) What is the average number of hours before test for the CPL?
4) Ditto for the IR?
Again, much depends on the calibre of the students, but at the very least you need to know this for your budget. Minimum hours are pretty meaningless - I would expect to budget at least 10% extra for safety.
5) If I sign up with you, are prices fixed for the duration of the course?
Worth knowing - if you're spending £20k on the CPL/IR, any increases in hourly costs (or fuel surcharges) will hit you hard.
6) If aircraft go tech, do you have contingency plans to source replacements?
If the school only has a couple of IR training aircraft and one goes tech, everyone will be fighting over the remaining one. A back-up plan at least shows that they have thought about the issue.
7) What are the backgrounds of your instructors, and for how long have they been with you?
A polite way of saying; "Are they career instructors or just treading water until they can get into the airlines?"
8) Do you have direct relationships with any airlines, i.e. do they accept recommendations for interview?
As discussed above - if the school has links with airlines, you have more of a chance of getting your foot in the door. Take nothing for granted however, not everyone gets recommendations.
9) If I am still unemployed after one year, do you offer discounts on IR renewals?
Pessimistic but worth considering. If you are still unemployed after a year, you'll be watching every penny.
10) Can I chat to some of your current and ex-students?
Probably the most important question of all. Get them out of earshot and you'll get the real low-down on how the school operates.