The problem with a Beslan operation and why I think it's unlikely is that it won't net the instigator good press with those they are attempting to influence. Symbolism is often more important for attackers than body counts (especially so for sophisticated attackers) and in Mumbai the decision on what to target was obviously based on symbolic value rather than maximizing a death toll. The symbolism associated with attacking a school and murdering children will not play well even among many Jihadists, so I think a sophisticated crew that can plan and carry out this kind of operation are unlikely to target schools - especially after Beslan, which didn't work out too well for the Chechens.