my teacher doesn't give out A's because she says "nobody is perfect." How is this allowed?

That’s absurd.

“Nobody is perfect” may be true; however, someone’s work product may be!  If you answer ten math questions correctly, that’s 100%, which is perfect.

Please give your parents these words as artillery to the parent conference they should demand:

Grading has been determined by the Board of Education. They are A to F.

The ranges are not subject to a single teacher’s arbitrary opinions.  If this teacher chooses to set an arbitrary score ceiling at 90?  Then that score must receive an A.

As a bonus?  Ask the teacher how they would feel if a new principal came in and said, “Only perfect teachers deserve full pay, and nobody is perfect!  From now on, your salary will be reduced by ten percent.”

easy she hasn't got a paper that deserves an A , when you get an A then its perfect .

You should report this to the principal or the appropriate authority at your school. Get your parents to back you up if they don’t listen. If you take an exam and you get every question correct, that’s a 100, or an A. Some teachers don’t give A+, which is fine, but that grade of 100% needs to count toward your GPA. The letter grades are irrelevant, but if you get all questions right on a test, that’s a 100, not a 90. That can negatively affect your GPA. If a teacher is admitting that no matter how well you do, the best you can get is a 90, then that’s unfair to the students who take her class. Getting an A isn’t about “perfection.” It’s about doing the assignment correctly. Some teachers grade on a curve, and your teacher is doing the exact opposite. Hopefully your school will recognize that your teacher is grading unfairly, and make her change her grading policy. Everyone who takes her class is at an automatic disadvantage compared to students who have a different teacher. 

One of your parents should report her to the principal.

Where are you? No school in the US would allow any teacher to operate this way. 

Complain to the principal. If you get a 90, that is an A. 

Perfect everything she throws at you! If you find that impossible then that's exactly what your teacher is saying.

That is... actually against the rules. I would speak to someone about this, in particular either the principal or the department head.

Talk to her boss, the principal.

That's a ridiculous premise, and you aren't wrong for being upset. That being said, don't get too hung up on grades. They only matter to the school system, not to the real world.