Wide clustered index vs multiple narrow nonclustered indexes?
If you will always have SchoolId then you could benefit from making the clustered index a composite key of SchoolId, Id as you won't have to have an additional index on SchoolId to avoid a table scan.  Not having the additional index will let Inserts/Updates/Deletes complete faster as the transaction only has to update one index.

You may find as you create other queries that use the other fields in the where clause that an additional index on those might be beneficial so SQL Server can seek right to that index leaf which will help reduce the number of row returned.