Hi there!
A decent level to end all subjects on is your target grade. If you hit it/ are above it, you absolutely don't need to worry
As a Y10 student, I can tell you that grades don't matter when it comes to choosing your options. All your levels are incredibly good, anyway, so it won't matter what you take because I can tell you'll ace it! Thankfully you don't require any level to do Triple Science. For the minimum grade, I'd say around a 6c.
Remember, all your past levels don't matter at GCSE. As long as you put the effort in when doing the GCSE course (from Y10 onwards), you'll be fine. All your previous years of high school are merely warm-ups, and you'll be learning completely different stuff at GCSEs anyway. I can confirm this as I'm doing Maths, English, R.E, Triple Science, Geography, Computer Science and French, and honestly, the only subjects you need past knowledge about is English, Maths and languages. You are excelling in everything, so once again you won't need to worry!
I'm afraid what your teacher said isn't completely true. Like I explained earlier, your past levels don't matter in GCSEs. They merely provide the basis for your targets, which many schools make a fuss of but they really aren't anything major because the majority of people go past them anyways. Your performance at GCSE is the real important thing here, so if you are getting around Grade 7-8 at a subject in your GCSEs then you are practically set!
Whilst it's handy to have a high level if you want to get A**, a level 8c doesn't mean that you're guaranteed to get an A**. Only around Y10 will the school truly give you an idea of what you should be aiming for.
Hope this helps! <3