To tell exactly what the problem is would require some direct observation, but we can make a check list.

Embouchure - the lower lip should be rolled out and relaxed, with the corners of the mouth low and the whole facing curve of the mouthpiece within the mouth.  There should be minimal pressure of the top teeth on the mouthpiece - no biting!  

Voicing - should be done with the tongue and oral cavity, not with lip pressure.   Think of making a  sort of hissing "eeeeee" sound to get your tongue position right.  That will bring the base of your tongue forward and open your throat up.  It will also allow you to control the airstream into the mouthpiece.

Playing stance - straight posture that will allow you to provide good breath support.  Have the strap adjusted high enough that you do not have to tilt your head down.  A down-tilted head makes proper embouchure more difficult.

Breathing -  fill the lungs and push inward and upward with your stomach muscles when you exhale.  That will allow you to support and control the air going into the horn properly.

Practice routine - MUST include long tones.  Be aware of all aspects of embouchure, voicing, stance, and breathing while practicing long tones.  Practice long tones in the range where you're having trouble and down to low Bb.   Once you can sound the notes out, work on control to make the long honks into musical notes with good tone.

Equipment - there could be one or more leaks in the horn, causing problems reaching the bottom of the horn.   That could be difficult to diagnose between now and Tuesday, so you might be forced to play through leaks.  You could also try a softer reed to make it easier to play the bottom of the horn.

Good luck too ya!