I got rejected from sixth form, but I have the grades I need to get in!

I think it's the D in maths, most sixth forms and colleges only accept you if you have grades A-C in maths, English and usually science.

Appeal?
Ask for feedback why you were declined?
Your reference? E.G - How is your behaviour, general punctuality, attendance?
Entry requirements for specific subjects? E.G some subjects require C minimum in Maths/English but would much prefer A*-B

Gotta give yourself a long hard stare in the mirror and ask yourself why.

Speak to the Head of Sixth Form and don't let them play you around. Ask why and argue for yourself :P Don't be rude or anything just give reasons (use your friend as an example for comparison) why you think they're wrong for not accepting you. Try to come to an agreement
Most Sixth Forms want C's in English and Maths, but many give options to resit.
Make sure to work real hard on Maths and show the teachers your improvements if necessary. You can do this! :P
Also apply to other Sixth Forms if you can so you have a backup.

I hope it works out well for you.

Thanks that really cheered me up! But I don't want to go to other sixth forms because they aren't really that good and my sixth is the only one who has medical science, which I want do! And I have also started revising for Maths and I revise everyday from 6-8pm and sometimes like yesterday I revised from 12am to 2am, thats how much I am determined to get my C in maths.

All you need to do is speak to them, and fight for your place (not literally :P ). My head of Sixth Form is really nice and she gave everyone a place, except for one person, but he doesn't have the requirements :P
Most people from my school aren't going sixth form anyway, and I am glad

But, you deserve to go, because obviously you're trying hard. Just show them that. They don't seem extremely demanding with their requirements so just make sure you meet them.

Good Luck

Is it possible for you to apply to any other 6th forms? If not you could try convincing your teachers that you can do it again, I am sure if they beleive that you will work hard to achieve the neccesary grades they may reconsider your application. And if worst comes to the worst and they say no, work extra hard to get the grades and then you can reapply next year Good luck!

Ok thanks, but do you think they will accept me with a D in maths, my friend got an F, and I will do resits to get the C in maths because the university wants a C too so.

I got an F in Maths and still got in to all the courses I wanted to do at sixth form. Could it be anything else apart from grades? Is your attendance bad etc? Your teachers give references to your sixth form normally and they were iffy about letting me in 'cause I had quite bad attendance in high school.

First of all, nobody is accepted until results day :P You friend may have got an offer on the condition that he/she will improve that F by resitting it.
Just study for your resit :P If they offered them a place with an F in Maths, they shouldn't deny you one even if you have a D. Just make a "deal" with them.
Your attendance and behavior seem fine, so I don't say any other reason why they wouldn't accept you.
What A levels did you pick, if you don't mind me asking?

I picked Medical Science, Health and Social Care and ICT

I behave very well and I get outstanding on my reports on behaviour and my attendance is 98.4% and they want six A-C including english and maths but when I asked them if they would accept me a D in maths they said yes in december, but why me OUT OF 450 PEOPLE?!

Did any of those have specific additional entry requirements, e.g. minimum of GCSE double science BB/ minimum of GCSE Maths B etc.
They are not allowed to discriminate against you for being foreign, unless you are out of catchment - i.e. are currently boarding at secondary school. They don't have to consider out of catchment applications until they have looked through and decided who to give offers/ rejections to for all of those who were in catchment.
Phone them up to ask why you were rejected, specifically mentioning that you were told before applying that your Maths D would not be a problem. However, think back - did they say that the Maths D would not be a problem in terms of you getting an interview, or that if you resat and the grade improved before the end of the year it would not be a problem? Have you entered for a resit? Do they know this? Are your teachers predicting that you will get a D overall even with resitting in June? They may have misunderstood what you meant about the D when you asked about it.
Also, perhaps bear in mind that with your current GCSE grades an academic sixth form would probably not suit you. Have you looked into other, more vocational, options? There are plenty of apprenticeships/ vocational courses/ on-the-job training available if you look - ask your school careers department for some advice.