Hy
I have a similar problem using Model Builder and Spatial Analyst Tools in Arc GIS 10.0 (ArcInfo Students License), but didn't find a solution yet.
The Spatial Analyst Extension is activated and enabled and works very well during Geoprocessing.
Now I've created a Model with several steps using Spatial Analyst Tools (e.g. Zonal Statistics and Raster Calculator) and everytime I execute the Model, after the first tool suceeded, I get the Error Message "ERROR 000824: The tool is not licensed. Failed to execute". What I did so far is just to disable and enable the Spatial Analyst Checkbox in Extensions again and execute each tool at a time. But I guess there must be an other solution? Else using a Model, which should be able to do this automatically, doesn't make much sense for me...
Hope someone can help!