The request for Miss Watkins-Singh to return to Aberdare Girls' School immediately was made by the London-based human rights group Liberty this week.
It follows a decision made at the High Court last month that the student's claims of indirect discrimination against her school should be heard.
Miss Watkins-Singh, from Cwmbach near Aberdare, claims the school's governing body indirectly discriminated against her by refusing to allow her to wear a kara, the bangle that forms one of the five Ks of the Sikh religion.
The student was excluded from the school in the South Wales Valleys last November after refusing to remove the bangle.
The governors at Aberdare Girls' said that wearing the bangle was against the school's code of conduct, which bans all jewellery apart from a wristwatch and a single pair of stud earrings.
Anna Fairclough, solicitor for Liberty, said: "The judge decided the harm to Miss Watkins- Singh being in school wearing a kara - because of the disruption it would cause - was greater than the harm to her of not being in school."
Ms Fairclough said she believed the High Court case against the governing body is not likely to be heard until late March or early April.
Miss Watkins-Singh is currently receiving home tuition.