Traditional thank you cards to a teacher

I want to write a thank you card to my Japanese teacher (in Japanese of course) and I happen to have 4 spare Japanese celebratory cards.

They are plain white, with white and red cords tied into a 'pretzel'-style shape at the center, with white and red folded paper flattened at the top in an intricate shape, which has a gold band around it.

Are these suitable or are they for a specific occasion? (i.e. Weddings, Birthdays, Condolences, etc.)

I'm worried I might offend her or end up looking foolish,

Thanks in advance,
Minho Charisma

The design is taken from the envelope normally used for putting the congratulatory gift of money for occasions to celebrate - such as weddings, anniversaries, children's graduation, etc. So.... not that it would offend or anything, but it's just not used for thank-you card. If I were you, I would go to a stationer's shop and find a "thank you" or "arigatou" card that you like for your teacher :)

Actually, if it's the "pretzel"-style shape, which I assume is what we call "musubikiri," it could be offending.

That knot is for celebrating things that are not to happen repeatedly. For example, you'd usually want one wedding in your life, so the "pretzel" would be appropriate on weddings. On the other hand, moments of thanks should happen repeatedly.

Either way, as AK suggested, a more plain envelope and paper is appropriate for expressing gratitude. Google 礼状用の便せん for examples. Or you can just write it on a simple hagaki.