"Are you ready for today?"
"Yes ma'am!"
"Sorry, am I talking to a swarm of mice here? I said, are you READY for today!?"
"YES MA'AM!"
"That's what I thought! Now, today's drills! You've got me today to measure exactly how much stamina - or maybe lack of it, frankly, that you think you have! So we're hitting the tracks and you're going to be racing!
"Now as it happens, the other Wonderbolts are using most of the courses on campus, so only one track's been booked for us, but the good news is, there's twenty-five of you and the track's large enough to race five cadets. So at least each race will be fair: five groups of five ponies to race!
"However! I'll personally be watching every race and recording each of your results! And by the end of it, a lot of you will have made it so clear, I said SO CLEAR, how way off you are from being a tried and tested Wonderbolt, that it wouldn't be worth the effort it would take to get you to race again!
"Get it now? Those who actually prove they can outfly a snail in a hurry, I might make you race again just to make sure. So don't you dare think about trying to take it easy just after one win. ANY OF YOU! Anypony who can prove themselves the best three fliers out of all twenty-five of you get lighter duties tomorrow. So I'm expecting a good showing if you care about that at all!
"Got all that, rookies?"
"YES MA'AM!"
"Good to know! Now head to the tracks, in an orderly fashion! We have a long day ahead!"
I assume she has no timer to just clock them, otherwise it just be 5
But if we have no actual way to compare other then "A is faster then B"
5 races can eliminate 10 of the slowpokes (the ones in 4th and 5th in those)
Then shake up the pairings so that all the ponies who got 1st go together, same with 2nd and 3rd(now at 8 races)
Now top 3 of round one 1st place finishers, as well as the winner from the 2nd and 3rd place races go, top 3 of that are the ones we want
Witch means 9 races
Judging by the lack of response, I assmue that answer isnt right
So ill take another approach
as before, split the 25 into 5 groups of 5 (5 races) this is round one
Then race those 5 winners, 1st here is 1st overall (6 races overall) this is round 2
Then we take the 2nd place from round 2 and the 2nd&3rd of the round one races where the 1st and 3rd of the second round came from,top 2 is 2nd and 3rd(7 races)
If we get really lucky -- 12 races: 5 of them determine 15/25 (because each time 5 ponies run only three may make it). Then have 3 more -- 9/15. Have two more -- 6/9 (as if out of 10). One more to leave just four (i. e. 3/5 + 1 that did not participate) and one more to leave just three -- twelve races determine the three fastest.
But, if she's got a stopwatch, it'll be just 5 races.