Do you use Remind101 to keep in contact with you students' parents? If you don't, you should definitely try it out.
This free, editable product contains two parent letters. One parent letter is for if you enter the parents' numbers yourself to add them to the group. This letter just gives them more information about the program. There is a second letter option also for if you prefer to allow parents to sign up themselves by texting a given number with a message. I usually have parents sign up for Remind during registration day and open house, then enter their numbers myself. For the parents I do not meet, I send home the second letter in hopes that they will join the group. There is also a sign-up sheet provided in the document.
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