From my experience, yes they do. At least initially. I like to think that after a couple of terms the teachers have cottoned on that they are actually different people though ;)
DS1 has ADHD and ASD and as such, could sometimes be a challenging student and certainly stood out in his class. DS2 is NT and 2 school years behind DS1, so when their Primary school started mixing year groups it meant that Miss X would have DS1 for 2 years, followed straight on by having DS2 for the next 2 years. Most of DS2's school reports had to be sent back to be corrected as they switched between names, parent teacher meetings went much the same way with the usual "oh 2 is really quite easy going isn't he?" comments at the first one. He is, but not more so than most kids, so the teacher was obviously expecting him to be more high maintenance because DS1 was.
In an accidental social experiment ;) I have tested this theory. We recently moved areas and schools and DS1 is in Seniors now while DS2 still has a year at Juniors. This is the first time DS2 has been in a school that hasn't had DS1 first, so he can be judged purely on his own merits (or lack thereof at times ;) ) and all of a sudden he is being described as lively, confident and outgoing. Previously the same behaviour from him was described as hyper, boorish and outspoken. Of course a lot of that may be due to different schools and teachers, but I do think a lot of it is down to him not being in his brothers shadow and the teachers not having preconceptions about him based on his brother.
This is one of the key reasons DS2 is going to a different senior school to his brother in September. Academically they are pretty equal, he has friends going to both, they are equidistant from the house but one will see him be known as DS1's little brother and one will know him just as DS2.
It was the other way round when I was a kid. My sister was 4 years ahead of me, so at seniors we had the same head of year and form teachers. She was a swot and a teachers pet. Library monitor, sound effects crew for the school plays, played in the brass band, the works. All through school I kept hearing "why aren't you more like DSis? DSis always handed her homework in on time/didn't talk back/wasn't caught once a week bunking over the gates to go to the shop for fags"