Dear Jessica,
I knew you want to buy a gift for your English teacher,Miss Green.She had been very helpful in teaching you English and you wanted to express your gratitude to her.You are procrastinating what to buy as gift to her.
Jessica,I would suggest you just make a 'Thank you' card yourself and present it to Miss Green.At end of the year,you can show your excellent English result to her.I think your good English result is the best gift for her
Sincerely,
sunanswerman