We'll start with an example.
One of my friends had trouble doing MX2, and they got a mark of 41/100 in the trials. What happened? Did their best, and they actually managed to scrape an E3 despite this.
This really shows only two things. Firstly, hard work pays off; even if it wasn't the best possible mark it was quite a jump from being in the fail range. Secondly, 4U is one of the only courses where scaling can save you (although any sane person would not rely on scaling to deal with this.)
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Now, let's look at your scenario.
Clearly you've kept up really high marks, most noticeably in English and 2U mathematics. If you feel as though dropping MX2 is a good choice, I definitely do not blame you.
Unlike Wales, I won't give you incentive to keep it because the decision is going to be your's at the end of the day. I want you to consider all of the following.
1. Seriously contemplate just how much you enjoy it. This is probably the most important question and I want you to FULLY consider how much you enjoy it.
2. Why did you choose to pick up Extension 2 anyway? Was it because you killed both Extension 1 and 2U in prelim? Was it because you wanted to challenge yourself? Or just a random choice?
3. What methods do you use to study for the course?
4. Information doesn't get retained that easily. I had that happen to me all the time in English and also quite often in physics. But forget about why that happens, more importantly how do you deal with it?
5. How beneficial is it? You are aiming for medicine, but as an example, what would be your backup?
There's no real need for me to push for reasons to keep, but there's no point in me giving you reasons to drop either. You should also consider compiling a full list of pro's and con's to dropping.
And, on a brief note regarding your teacher's suggestion. The teacher isn't wrong; staying in MX2 will help MX1. But you have to weigh the costs v.s. benefits. If it were me, I find that I study better when I am full aware that the exam block is starting within 4 weeks. So for the most part, I wouldn't have cared about the time lost if I were to drop, because I'll just drop 4 weeks before the exam block. Again, consider the costs v.s. benefits. (Timing may be useful.)