I like to be at places extra early to get settled in and calm down. If the class start early, I would be a bit annoyed because my mind is programed for class at 9am, not 8:55am. I'm certainly not waiting for anyone (per the last line of the OP). Witnessing that exchange would not make me impressed with the instructor; like she just wants to hurry up so she can leave.
This was my thought, too. I leave that bumper of time for mishaps, but I also like to arrive early so that I can prepare myself. I use those few minutes to use the restroom, sharpen my pencil (I guess we don’t do that anymore), review any related notes, or even just to social/network with others. It’s not a huge deal that the instructor started early, but I would be especially annoyed to be that last person with NO time to settle in. While I would still manage, I would certainly not appreciate being told that I was late. The OP was NOT late and should not have been treated as if that was the case!
This reminds me of a flight I took once. I had planned enough time to grab a coffee and breakfast prior to boarding the plane. When I got to my gate (breakfast in hand), the flight attendant acted somewhat impatient with me.
Flight Attendant: “Oh good, you’re here, we paged your name, but you didn’t show up.”
Me: *slightly panicked* “I’m sorry; I never heard it. I didn’t know that I was late!”
FA: “No, you’re not late. The pilot just wanted to pull away from the gate early to avoid a long wait on the tarmac.”
I felt embarrassed, but I wasn’t late. The funny this is that I really didn’t hold the plane up, because there were others still being seated. I felt a little guilty for eating my yummy breakfast, though.