There's no way I could have a vehicle because I am too young for any rentals...

Oh well. That complicates your logistics, but that's about all. And realistically, it puts certain parts of the state
out of your reach.

To put it in the simplest possible terms, there are only so many days you can be on the trail before you need to reload with food and fuel. There's only so much room in your pack, and only so much weight you can carry before the burden of it takes all the fun out of the hike. Too much weight causes stress injuries, too.

So, for each place you decide to go, you'll just have to figure out where you can buy supplies at intervals, whether it is on your way in, or an interruption in the middle of a long hike. Wilderness being what it is, it is generally not very near to roads, towns and supplies.

This is far from an insurmountable problem, but it is a real one and you'll need to have a strategy worked out for it - whether that consists of finding a trail-rich area not too far from a town and exploring it in a series of moderate-length hikes with resupply runs in between them, or taking a long hike, with predetermined spots where you can leave the trail and get to a town, then come back to the trail again. Or something else.

That second strategy (a long hike with some jaunts off the trail) works very well for places like the Appalachian Trail, but is a bit trickier in Oregon, where the population is thinner, the wilderness is wilder and is more remote from towns.

In deciding on your strategy, it will be important to understand how much you can carry and how far. It will be easy to miscalculate, because you haven't done it much, yet. So, even if you have no hills to test yourself on, try to borrow a pack and start carrying loads, so you get the feel of what it is like and get some kind of gauge on yourself. The difference between 20, 30 and 40 pounds is not a straight smooth line. At some point added pound of weight starts to affect you more drastically than earlier ones did. Where that point falls for any one is not predictable.

how long does the bus ride take? That seems like quite a ways, from Portland to Ashland

It's about 300 miles by freeway. But I do not advise your trying to solo hike the Oregon section of the PCT. You'd need to go into intensive training and planning and you are still in school and couldn't devote enough time to be prepared for that in the time you have. You might manage a shorter section - possibly. You haven't taken enough baby steps, yet, tbh.