After the Rangers retreat, the footage of Brax is flipped left to right.
The Ninja Steel Megazord and the Ninja Blaze Megazord cockpits only have three podiums each. Normally, even when there are absent Rangers, all podiums are present.
From Sarah's statement at around eleven minutes, only a day passes between the two math tests. Each time it would seem Mrs. Finch grades them before returning them (as shown on the first day). Even if students exchanged papers to grade one another, Mrs. Finch would need time to gather them and record the results before handing them back (with students sitting around waiting for their result). That's a lot to get done and a bit of wasted student time for the usual forty-two minute class time allotment.
Apparently, due to the empty cans filling the math class trash basket, VIctor and Monty ate beans in the classroom just before the test. This presumes that there was no class ahead of theirs and that the door was unlocked. It still doesn't explain why they'd be stupid enough to leave evidence of their planned fart sabotage in the same classroom.