The past tense... Good choice. Everything sounds so much more certain. That certainty lends a lot to the emotion of the scenes. You're wielding the past well.
The final scene is great. There's mucho conflict, a misunderstanding, deception, and helplessness-- all great things for a character to grapple with in a good drama.
One thing that would bolster this even further would be to tighten your POV. I'm a broken record, I know, but a tight and strict POV is essential for this type of story. Some examples of POV meandering (and ways to repair it):
[--“S-s-sorry. I j-just wanted… never mind.” Annabel was nervous enough as it is calling the girl without that news startling her. She moved to hang up, but Riley’s voice stopped her.--]
We started this scene waking up with Riley. Then we're in Annabel's head, then back to Riley's, and back, and forth...
Try to convey Annabel's feelings indirectly--
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“S-s-sorry. I j-just wanted… never mind.” Annabel sounded nervous-- more than usual.
Riley spoke quickly, before the skittish girl might panic and hang up.
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Keep that up throughout the exchange, and we never leave Riley's head, yet we get a sense of Annabel's emotional state. In a scene like this (telephone,) we're not just hopping heads, we're hopping locations too. Very disconcerting.
[--There were men, boys, women and girls of all ages. Some short, others tall. Riley listened to the professor drone on.--]
This is a bit too vague to even consider including. You could get more specific to paint the crowd--
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Kids from the inner neighborhood, the idle wives of too-rich husbands, a few doddering senior-citizens with too much time on their shaky old hands...
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You see how that gives a sense of a variety of specific generalities (oxymoron, I know) rather than vague generalities?
Great story you've got here. Your characters and dialogue are strong. You're sense of drama is pretty keen too. Keep up the great work, and embrace the past tense. It suits your story and its emotional content very well.
Looking forward to more...
-SQ
Thanks a whole bunch.