The TPS has previously been valued (last time was in deficit, despite strident voices asserting otherwise), and does include in it's existing terms, the provision for increasing contributions. It's not an unrealistic scheme.
But teachers need to accept that the longevity time bomb applies (look at the actual stats - longevity us rising, and teachers aren't as short lived post retirement as I have sometimes seen claimed).
The Unions state that the scheme is cost neutral to the tax payer, and that therefore my last two paragraphs are wrong.
Great!
There's a really, really simple way out!
Remove the unlimited guarantee that the taxpayer will pay any future shortfalls in contributions to liabilities. Essentially, tell the unions that their position is so convincing that in future the money will be where the mouth is. They could still use publicly funded officials to run the scheme (no new costs), and need not build up a fund (pay into and be paid from general governmental funds), but either pensions would have to be variable (to match contributions - I'd see this as unfair, but those convinced there is no deficit wouldn't have a problem as they are clear that is an impossible scenario; or the contributions have to be variable.