What is to be done given the current circumstances in Tunisia, where the loose, long, Sharia-compliant hijab may cause problems in mixed-gender institutes and schools due to its social unacceptability there, and the difficulty of finding suitable sizes for girls aged seventeen, knowing that the common dress for veiled girls of this age is not Sharia-compliant?

It is obligatory for you to continue wearing the lawful hijab (definition) and not remove it under the pretext of society's rejection of it, for patience in obeying Allah is a duty. The path of righteousness inevitably involves trials and tests to distinguish the truthful from the false. Allah tests people with ease and hardship to know who remains steadfast in their faith and who is swayed by doubts and desires.