I am a "little older" too, as is LIS, and I agree with him. In a large lecture hall where few instructors know more than a handful of students, your dress does not matter unless it is offensive. Students are students...whether MBAs, med, vet pHd, I have never seen dress matter in a classroom. In a job environment or clinics when dealing with clients especially ....that is a different story.
Agreed.
In clinics where you are interacting with clients? Sure.
Sitting on your butt being lectured to in class? I see no reason to be uncomfortable. Now this doesn't mean dressing in tube tops and stuff like that, but nothing is wrong with jeans and t-shirts to me.
When I went to VMRCVM there was no dress code besides a warning against things that show too much cleavage, etc. Dunno if it is still the same. I see many of the students here at A&M dressing more professionally, but that's likely because of the requirement.
Maybe it's because I'm a little older, but I have no problem with a dress code at a professional school (aside from being irritated that someone is telling me how to dress... of course that's probably because I've been in the military for so long). In my first two years of undergrad in CA,
there were students that I felt embarrassed for due to their clothing choice for the day that consisted of 4-day old jeans, old rock band t-shirt, and flip-flops. If you want your classmates/professors to perceive (keyword) you as a young or old, immature, lazy individual by merely looking at your clothes, then I can only imagine the perception of a vet school student in sweatpants and a t-shirt.
Now I'm not unrealistic to realize there will be those days where you feel like death or pulled an all nighter, but as a general rule...
You're paying a school a LOT of money to be a professional, a doctor, and presumably your dream... so act like you care and show a little class. 
I dressed in jeans and sweatshirts all through my first three years at vet school (with the exception of special speaker days), and I wouldn't care a whit if you "felt embarrassed for me"
Your professors too - they aren't your bosses or employers - you are
paying them to teach you. Earn respect by your
performance. Hell, I often wear jeans and sweatshirts as a resident now, unless I'm giving a talk or on the floor. If anyone dared to suggest I was "immature" or "lazy", I would simply laugh and point out that due to my performance I'm in one of the top programs in the nation. And I'm not saying that to brag, I'm saying it to make a point. I "acted like I cared" and "showed class" through my work. That will gain you MUCH more respect than any clothing choice.
Now, I'm not saying to show up covered in filth, but there is nothing wrong with casual clothing in a school (keyword) environment. The first three years of vet school are a
school environment. Not a
professional environment, and miles away from the real world. Leave that for clinics.
I don't care what my first-semester anatomy prof, as much as I may respect them, thinks about my clothing choices. I care about what my clients or employers think. Professional school does not equal professional career.
Just my 4-day-old-jeans-and-rock-band-t-shirt-wearing opinion.
Capirinha - I also have some visible tattoos and dyed my hair several, ahem, "unique" colors during school. I only bothered to hide them during clinics, just in case I was working with a particularly conservative client.
