Found this on another forum:

Save $70 per line at T-Mobile on top of existing T-Mobile offers (see flyer for all details). To redeem call the toll free number 844.786.2964 and mention the promo code STUDENT01.

Looks like this offer is only available via TOLL FREE NUMBER and not available online. Seems like it works for everyone since I am not a student and it worked for me! I think this only works for new lines/add a line but not entirely sure.

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$70 off per line offer detail:
Get a free, bonus $50 prepaid MasterCard® with each new line of service you activate on T-Mobile One or Simple Choice**
Save $20 per line: waived SIM kit fee for each new line on T-Mobile One or Simple Choice
Your prepaid cards can be used for T-Mobile bill credits, or anywhere MasterCard is accepted
Get the best offers, and save an extra $70 per line!

**$50 additional MasterCard and waived SIM fee offer only available by calling the toll free number in this deal to purchase and activate. Shipping is fast and free.

See More: Save $70 per line on top of existing T-Mobile offers