How to send a text through your email client such as Outlook:You simply need their cell phone number and mobile service provider such as AT&T, Sprint, Altel, etc...
Parents and students need to know the standard disclaimer "standard text message rates will apply"
Here is the formula you need [10 digit telphone number]@
How to send a text through Twitter:
In the last six months Twitter has introduced "Twitter Fast Follow." This service allows anyone with a cell phone to receive your tweets as text message. Teachers would need to create a twitter account and then parents and students could subscribe by texting Follow @teachersname to 404-04. Here is a link to Twitter's easy to set up guidelines. Twitter Fast Follow