I recognise this situation as in my workplace it’s very common for students to get assessments privately, and sometimes without informing us first. You are then put in a very tricky situation!
In the past, I have requested that the private assessor complete the Part B of the Form 8 (if the scores they have reported meet the JCQ criteria), and I have been able to discuss with teachers and agree that this is a justificable and reasonable adjustment. However, to go with this, you still have the challenge of writing up a history of need, current difficulties and normal way of working. If the need for extra time has not been recognised or supported by teachers before now it is very diificult to do this, and on the back of that, I would very kindly be saying, sorry but this is very last minute and we don’t have any evidence to support this arrangement.
Unless it was an exceptional circumstance, such as a student had recently joined the school or teachers were raising serious concerns about timing I really wouldn’t agree to attempt to assess them this close to the deadline.
Is there evidence in the dyslexia assessment report that meets the JCQ criteria? Happy to help if I can and would be interested in what others do or have done in similar situations.