Do the trunks have special compartments or something? How do they fit all of their school supplies, casual clothes and personal belongings in them? They have big school supplies like a cauldron and a telescope. How do they fit all of their stuff in their trunks?
Hogwarts trunks come with Extension Charms already on them.
The J.K. Rowling writing on Extension Charms states that the school trunks at Hogwarts have Extension Charms built into them, which both make the trunks bigger and the contents lighter.
Hogwarts school trunks, like the majority of wizarding luggage, are issued with capacity enhancing or extension charms as standard. These spells not only increase the interior dimensions of objects, while leaving the outer ones unchanged, they also render the contents lighter.
- Extension Charms (Pottermore)
The students do not cast the charms on their trunks themselves - casting Extension Charms on objects has to be approved by the Ministry of Magic.
The Extension Charm (‘Capacious extremis!’) is advanced, but subject to strict control, because of its potential misuse. Theoretically, a hundred wizards could take up residence in a toilet cubicle if they were sufficiently adept at these spells; the potential for infractions of the International Statute of Secrecy are obvious. The Ministry of Magic has therefore laid down a strict rule that capacity-enhancement is not for private use, but only for the production of objects (such as school trunks and family tents), which have been individually approved for manufacture by the relevant Ministry Department.
- Extension Charms (Pottermore)
The trunks are still fairly large and difficult to move - but they are indeed bigger on the inside.
It seems like the kids just have a lot of luggage. Harry uses a luggage cart at King's Cross, and he can't even move his trunk without help. There's no reason a telescope and a small cauldron couldn't fit in a large trunk with plenty of room to spare. If it doesn't, get a bigger trunk. You don't need magic for everything.
From memory, students accomplish this through a combination of three things:
1. Make the belongings smaller
Telescopes are collapsible: they can be shrunk down to fit inside an ordinary suitcase and then extended out to full length when used. According to Pottermore, and I'm sure it's also mentioned in the books, you can also buy Collapsible Cauldrons. So those two items wouldn't take up quite as much space as you think they would. (I can't remember whether there's an actual Shrinking Charm, or anything of the like.)
2. Make the suitcases bigger on the inside
In Deathly Hallows, Hermione uses an Extension Charm to make her purse bigger on the inside, so she can carry a disproportionate amount of stuff inside it. It's never explicitly stated that any of the students do the same thing with their suitcases, but it's possible. Or failing that...
3. Bring more suitcases
Again, I'd love to get a screenshot, but I'm pretty sure the early films show the students at Platform 9 3/4 carrying about half-a-dozen suitcases each. So if they couldn't fit all their stuff in one trunk, and they didn't know the Extension Charm, they could always just buy more trunks.