Can one continue to communicate with a female classmate who forbids communication, claiming it is impermissible, while one wishes to propose to her in the future, and fears losing her or disobeying Allah?

This relationship must be severed, and it is impermissible to continue it under the pretext of fearing the loss of a marriage opportunity. If you are currently unable to marry, then occupy yourself with what benefits you in your religion and worldly life until you are able to do so. There is no objection to proposing to her guardians now, provided that she waits for you until the expenses of marriage become easy for you. However, no relationship between you is permissible, whether via phone, internet, or otherwise, until the legal contract is concluded. And whoever abandons something for the sake of Allah, Allah will compensate him with something better.