Asking a teacher for a recommendation can be daunting, especially during the hectic transition between Junior and Senior year.
I recommend asking teachers for a recommendation during the spring of Junior year, then following up at the very beginning of Senior year.
Keep in mind that recommendations are a lot of unpaid work for teachers, so be grateful for their efforts during and after the recommendation process
(at the very least, a thank-you card is in order).
Below is a helpful questionnaire you can provide for your teachers to help them in the process of
writing you a recommendation. You can provide this questionnaire, along with an updated resume
(take a look at this
workbook and
video
on how to write a resume),
to your teacher before they need to start writing your recommendation.