Hi Haggy,
If the picture is accurate, the brush marks used to apply the sealer are still very evident, suggesting the whole end of the piece needs a lot more sanding than just 'flattening between coats', also that this is not a turned piece of wood. The sanding sealer already applied should have filled the grain, and, once fully hardened, prevent the fibres moving when you sand them. However, if there are still areas that are patchy, (is that what the arrow is pointing to?) the fibres are still unsupported and more sanding sealer will be required. Leylandii is quite an open grain wood, and end grain of any wood will soak up large amounts of finish. The advantage we woodturners have is that we are usually dealing with smallish areas of end grain, and they are also relatively thin, so can't soak up that much. Your students piece looks to be thick and is all end grain.
One other thought, did you apply the sanding sealer neat, or was it thinned down? Most turners apply it thinned down, especially the first coats, to make sure it gets well into the wood. If it just sits on the surface, it would be relatively ineffective.
Hope that helps,