I think "shall" should be accepted for the purposes of this exercise; however - be aware of some differences between "shall" and "will":
-When "shall" expresses the simple future, it sounds more formal than "will" and a bit archaic; you are unlikely to come across it in everyday conversation or informal writing.
-In question form, "shall" is used to express an offer or suggestion: for example, "shall I call you a taxi?" (this is also a bit formal and stilted - in most everyday situations you would say "can/may I call you a taxi?" instead)
In most cases, you are better off using "will" unless you are sure of the tone that you want to convey using "shall".