I don't actually know your area of education but I work at a small high school and I was in a similar situation when I took over here many years ago and without sorting it out this are going to go from bad to worse. I found that documenting the problem and publicising it to stakeholders was the best policy - talk to pupils, parents, staff, governors anyone that has an interest.
The thought of an online exam failing half way through should make your SLT and exam officer go weak at the knees so perhaps they are the best people to concentrate on. Hope this helps.