Unfortunately in situations like this, particularly in speeches, it is at the marking teacher's discretion to determine the marks. You can approach your head teacher, but it won't guarantee a remark of any sort. I would recommend doing so though, just in the hope they might consider it. You may want to confront your teacher about it and she may just explain why your mark was significantly lower. When it comes to English, even though the content may be very similar in essays, the manner in which it is articulated is equally important. If she believes something as simple as sentence structure detracted from the ability to understand or coherently communicate your point of view, then she is within her rights to simply not attribute marks for it (you don't consider it as a deduction in marks in English as teachers start at 0/20 and try to award you with as many marks as possible) even if your content is the same. This is really the problem with the English subject, in that it is so subjective. I have found myself in similar situations such as you, but it is really the teacher's decision of how they think your writing is regardless of what you think you deserve. But to be frankly honest, the marking system at your school seems a little out of balance. The way they marked speeches at my school in year 12 was in front of a panel of 3 teachers and then they would take the average of the 3 marks. It seems more fair that way, and if there really was a huge discrepancy between the marks, e.g. 12/20 but the other two gave 18/20, the lower I think was disregarded or re-examined and the head teacher would decide what to do about it. So at the end of the day, it doesn't matter what you think you deserve, it's what the teacher thinks that matters. They're at no obligation to change the mark because you think you deserve higher, but it's worth a confrontation just in the off-chance it happens.