Polite way to underscore a firm deadline?

[browncow]

I am applying to a school which will not process my application if any letter writer submits one second too late. I've sent a reminder with date and time, but any suggestions for how/when to begin signalling greater urgency? At least one of my writers is somewhat disorganized.

[Loric]

Phone call, and if there's a secretary recruit them as your advocate.

[silver_lining]

Dear Prof. X, 

I appreciate the fact that you are writing a letter for me, but as I know you are very busy, I would like to remind you that a strict deadline for school X is on Date X. Let me know if you need any info to complete this task. 

Sincerely,

Y