Some guessed figures:
Tomo and Chitose are 15 or 16, having just entered senior high school.
International door height is 6'8". Tomo looks tall enough that standing on her tiptoes will bump against the door's head but just barely (i.e. she won't be jamming, pressing, stabbing her head into the door's head if she maintained stood up as tall she could). I would guess Tomo 6'5" or 6'6".
Chitose stands at about breast height (slightly above: 1 inch or so?) with Tomo. Searching the Internet for height differences like some Zoomer-Millennial border marches autist pretending to work (100% better than the gossiping middle aged housewife crowd, I swear), a good reference photo is LeBron James with Savannah Brinson at 6'9" and 5'7". Assuming Brinson is wearing 3 inch heels in the many photos of her at James' chest (really can't tell because photographers always cut off the lower body), this would make Chitose 1'5" shorter than Tomo at 5' or 5'1".
There are many assumptions in the above, not least of which is that the door is built to international standards and not some weird superior hot blooded virile Japanese ritualistic construction. Of course, the simplest explanation is that the author didn't think about these matters. Either way, Tomo and Chitose could be shorter. This is probable, because Chitose could barely lift a cardboard box filled with the type of supplies you would find in a school and looks to have normal proportions.