Give the teacher a certificate that doubles as a card. It can be a generic certificate, but get each student to sign his name. If the class is small, then each student can write a little note to the teacher as well. Fold the certificate in half, or roll it up and tie a red ribbon around it when you give it to her. Include a paper with the certificate explaining your gratitude.
Get a greatly oversized card so each student can write a special note to the teacher. You can find these at many different stores, but for a specific teacher appreciation card you may need to go to a store that specifically sells cards. If you can't find one, then get a blank oversized card and write something like "We Appreciate You" or "Thanks for all your hard work!" in the middle of it and let everyone write their messages around it.
Let each student trace her hand and cut out the shape. Then on each finger the student will write something that she appreciates about the teacher. If the students are young then you should do this with them so that all the things are written correctly and are appropriate. Each hand is its own individual card.
Have each student get a postcard and send them so that they arrive during Teacher Appreciation Week. If you can, have the students all turn them in to you at the same time so you can just give them to the teacher in her box. This saves on postage for the students and ensures that all the cards arrive on the same day.
If you have a computer class at your school, then talk with the instructor and ask if the students can take the class time to make e-cards for their teacher. This is eco-friendly since it uses no paper and allows students to be creative and each pick their own card to send to her. Her inbox will be flooded with sweet cards from her students.