At school, I was taught that 'shall' indicated future tense with 'I' or 'we'. 'Will' was to be used with the second or third person, 'you' or 'he'. In the future perfect tense, it indicated a degree of confidence above future conditional.
..I think if you want to make it an order, then 'is to' / 'are to' would offer a bit more clarity of intent.
..it's a bit tricky as the use of 'if' slides you into the subjunctive mood, not a solid base to work from.
wets