The victim was on Wharf Street waiting for the bus inside the shelter opposite B and M at around 4.30pm on Tuesday when the two culprits approached her.
Police say they threatened her with "a bladed article" and took her phone.
A spokesperson for West Yorkshire Police said: "The victim managed to retrieve her phone but is understandably shaken by the incident.
"The suspects then crossed the road to B and M and walked up the main street with a number of other youths."
The pair were described as white males in their early teens, one with short brown hair and one with short blonde hair.
Both are believed to be wearing dark tracksuits.
"It is believed there may have been people at the bus stop at the time of the incident who may have valuable information which may assist," said the spokesperson.
"Enquiries are ongoing and anyone with information is asked to contact Calderdale District Police via the 101LiveChat function on the website or via 101 quoting crime reference 13220282405."
Information can also be passed on anonymously on 0800 555111.