Without sufficient cash to back the deal, heads warn, schools will find it hard to implement the reforms.
All parties rightly point out that the agreement heralds a possible breakthrough. Guaranteed marking and preparation time has been sought by teachers' leaders for years. Other elements, including the pledge that teachers should only "rarely" cover for absent colleagues, and moves to guarantee to school managers set-aside "leadership time", have real potential to change teachers' working lives.
But they also have staffing implications. With few primary teachers getting "non-contact" time now, schools will need more support staff or teachers to cover for them. This is why heads are so concerned about funding. Despite unprecedented investment in education, many areas are facing standstill budgets or worse this year, they warn.
Over the coming months, heads, local authorities and ministers will continue to haggle over the figures, with the Government pointing to a pound;3 billion real-terms increase in overall school funding by 2006.
But why has the NUT not signed up? It says it refuses to be bullied into promoting a deal that expands the role of support staff, something its members oppose.
However those who have signed up calculate it is better to influence the talks than be outside them. With the biggest changes two years away, they feel they have time to negotiate and ensure they are done properly.
The National Association of Schoolmasters Union of Women Teachers even suggests its monthly meetings with civil servants to monitor workload give it more influence than it has had since the end of collective bargaining in the 1980s.
So will the NUT regret staying out of talks over the enormous issue of teacher workload? Time will tell.