It is your trainers decision as they will know when you are ready. You don't need to be perfect at a lower height because perfect doesn't exist but you should be putting down consistent clear, smooth rounds as well as placing at shows.
Show them that you can do the height you are at now with no mistakes every time. Just because you are jumping higher doesn’t not mean that you are a better jumper. In the end it should not be your trainers decision what height you show at :)
Hey! You can gently ask why you're not jumping higher, your trainer will explain you the reason! Don't rush to jump higher just because you think you can, practice a lot till you know you can "win" the class
Hope it helps
Remember, height doesn't equal better, so don't rush to jump higher, make sure you have solid flat work first. It's the foundation to jumping and very important but unfortunately overlooked by many as well. Just be patient and have fun! Your trainer will know when you are ready:)