See this link for some instructions here for loading the pass at the fare machines once you have activated the One Card. Scroll down about halfway through FAQ on the page. I also inserted the instructions below. My teen tried to to this yesterday (before she read the instructions) and found it confusing and will try again on Monday.
http://ddot.dc.gov/DC/DDOT/Services/Transit+Subsidies/DC+Student+Passes#13
RENEWAL STEPS:
To renew your monthly Bus/Rail Pass, you must already have an unexpired monthly Bus/Rail Pass on your DC One card. You may renew at a Metro Sales Office or at Farecards & Passes Machines. You may renew your pass at any time in the month when you have an unexpired monthly Bus/Rail Pass on your DC One card and the new pass will be valid for use in the next month.
RENEW YOUR REGULAR SESSION MONTHLY PASS AT FARECARDS & PASSES MACHINE:
Touch your card to the card reader (circular target) at Farecards & Passes Machine. Press the C button: “RENEW PASS”.
A SMART BENEFITS
B ADD VALUE
C RENEW PASS
The next screen will show the student pass. Press the B button: “DC MNTH R”
A
B DC MNTH R
Follow the prompts to purchase the pass and add it to your card.