It is very unlikely that the University would be able to offer further immigration sponsorship for a Student visa extension for you to complete your reassessments as it is not necessary for you to repeat full-time teaching in the UK.
If you do not pass your reassessments and get your results confirmed before your current visa expiry date, you would not be eligible to apply for the Graduate Visa.
We are required to have strong justification for offering further immigration sponsorship for a resit period, which is why we do so only in exceptional circumstances, where full-time repeat study is academically necessary, attendance has been good and the student has experienced exceptional circumstances. This is confirmed in our Limitations to Immigration Sponsorship.
The University does not support 'bridging' visa applications under the Student route where students intend to apply for the Graduate Immigration Route, but have outstanding reassessment/repeat periods of study which are external or 'not in attendance'.