I agree with much of what has been said above. I personally feel like a clear comparison of the advantages and disadvantages would clarify your perspective further and thus enable you to make an informed decision.
- The test carries a weighting of 20%. While this should definitely not be ignored, a 20% weighting is not very significant and will thus not affect your rank substantially. However, it is important that, in the event where you don't perform favourably in this test, that you ensure you perform to the best of your abilities in your subsequent assessment tasks/exams that constitute the remaining 80%.
- You seem to have a study plan ready for Chemistry, involving teaching yourself the content during the holidays in addition to getting a tutor, both of which can be very beneficial and potentially allow you to achieve the progress that you are aiming for.
- You have the determination to do well in Chemistry. This is perhaps the most important factor as it constitutes the basis for your studies, i.e. having the will to study and achieve great results and showing serious determination in doing so will enable you to reach your performance standards.
I will note that I don't really perceive any significant disadvantages from the information you have provided, although I will note the following:
- You are struggling to understand the content. While this may not necessarily be fatal since, in most cases, this kind of problem can be rectified using an appropriate approach (which is definitely something that you have thought of considering your holiday plans), it may be perceived as a warning sign that this struggle could potentially develop, resulting in unwanted performance-related repercussions. However, it is definitely safe to suggest that such an outcome is a bit extreme as of right now, especially since nothing has been decided yet.
You should not feel overwhelmed by the fact that approximately half of your class dropped as this in no way indicates your own academic potential. I strongly feel that the advantages of keeping Chemistry outweigh the disadvantages of keeping it. Based on this, I would suggest keeping Chemistry.
I hope this helps!