The local authority is the only body that can grant an extension and there need to be very good reasons for doing so. The school has a duty to identify as early as possible if a NQT is in danger of not meeting the standards and, to paraphrase the NQT guidance, “it should not come as a surprise late in the NQT year”. If the mentor has simply increased the number of observations without providing any other support, then that is not how to deal with any issues.
Read the NQT statutory guidance which tells you what should be happening when and how and who is responsible. If you are further concerned then contact the LA adviser for NQTs yourself and talk through the situation.