1. As a colt, Jupiter Sky was much like his father in the regard that he only really made friends with a few creatures, didn't really like the rough-and-tumble games many colts would play, and would much prefer to stay in and read. As he gets older, his slowly maturing personality allows him to make friends easier with most creatures. However, as a kid, he spent many a night in Princess Twilight's castle, subjected to tea parties and hair braiding by his two "best" friends, Princess Dawn Star and Athena. He found himself often playing the role of the terrified prince that needed saving from the sneaky draconequus by the brave princess warrior, or various villains their parents helped defeat. While he's not exactly silent with his protests, Jupiter still endures their silly little games, sometimes even with a smile.
2-3. Dawn and Jupiter both attended Celestia's School for Gifted Creatures during their school days (Athena split her time between Canterlot and Ponyville). Dawn was mostly taught in the ways of controlling and refining her magic, and the emotions tied to such. Jupiter's talents, however, were more in-line with his mother's than his best friend's. Incredibly studious, there were many ways in which Jupiter stood out from the crowd. That earned him many admirers, but also a few hecklers. Dawn bore the brunt of the sass and harsh remarks that were thrown their way (due to the combo of sketchy magic and princesshood), but Jupiter's snark matched that of each creature at that school. While the expectation was never there, there were many days where he would suddenly just show up in the hallway she was walking down, chiming in with, "its interesting to see you giving the future princess grief over her magic, when I noticed your incredibly dismal score as I was helping Professor Whitewing grade last weeks' test."
4. "Interesting stance on Starswirl's Theory of Relative Teleportation, but I gotta tell you…your main point could use a little tweaking to sound…oh, what's the word…better."
"Hmph, funnily enough Jupes, I was about to tell you the same thing."
Being the only two of Twilight's court to go to school in Canterlot, they of course bonded closely over studying, tests, homework, walking around the grounds on lunch, and being an anchor of support when either of them needed it. They know there's quite a bit they need to be ready for here in the future, but they'll be there to help them through each and every situation right?