Explaining a (possible) grade drop?

I am currently a senior taking four HL IB and AP courses (two of each). As of now, I have been accepted to five of the nine schools I applied to. However, a majority of the schools that have accepted me have stressed the importance of maintaining my grades in order for the decision to be retained.

My biggest concern right now is that my admissions offer (to whichever university I decide to attend) will be rescinded after I send in my final transcripts. Of course, this is a very negative expectation for myself and I obviously do not want to have to face this type of situation, but my maths teacher recently emailed my parents. She told them that although I was performing well in class (participating, working with my classmates, solving worksheets/problems) I “struggle with reflecting that on tests, which indicates a lack of preparation” – which is true. I have always had a problem with exam situations because I tend to overthink everything, which leads to me panicking and losing concentration.

I thought I had recovered from this “stage” of my academic career (my freshman and sophomore year grades were disappointingly bad because of this, but I showed improvement during junior year, which is what I believe led to my conditional acceptances) but I guess the stress of being in my final semester of high school has finally hit me. Although I do understand the material and can correctly solve related problems during class, I am somehow unable to display this level of understanding on tests.

So, to make a long story short – I am worried that my poor performance on maths exams will possibly lead to a rescission of the acceptances I have received so far. If this does happen, what would be the best way to explain it? I am trying my best to improve my test-taking skills, but I am worried that I will not be able to showcase my understanding during the official external exam.

Did colleges specifically accept you conditionally, or did you got a standard acceptance letter that talks about the need to maintain your grades? What did they say your grades need to be?

@halcyonheather - One of the universities specifically accepted me conditionally, two mentioned that I need to maintain my grades in order for their decisions to hold, and the remaining two sent me normal letters!