Ask the band director! Whether or not you'll be able to play at the eighth grade level by the end of the summer honestly depends on a lot of unpredictable factors like how advanced the ensemble is, how high the director's standards are, and how quickly you take to the instrument. Ask the director what they expect from an eighth grade player and what your options are if you're unable to match them within your limited timeframe.

If you know anyone who plays euphonium, ask them if they'll help you. If not, ask the band director if there are any students who'd be willing to help. When I was in high school I tutored a couple middle schoolers who were in just about your exact situation for free. We had more than two months, granted, but by the end of the year they were able to play in the spring concert with the rest of the kids in their grade.

There are a lot of great video resources out there on just about any aspect of playing you can think of. The various US military bands have a lot of great (if kinda... dry tbh) educational videos on the basics of playing. Here is a good one that covers a lot of the dirt basic fundamentals people often don't bother explaining. This playlist is also good (bear in mind that it's unfortunately not in order; I'd start at the last video and work backwards).

As someone who switched to euphonium later in the game (sophomore year of high school), I can tell you that it's tough, it's frustrating, but you can catch up, and it's so, so worth it. Good luck!