Now I want to Add all the GLH that are < 300, between 300-340 and 350+
so output will look like
However, if you have to have the data in that order, then that's a little more complicated:
In both answers, what we're doing is using a WITH statement to create a CTE (Common Table Expression). This allows us to create an intermediate pseudo-table which holds the results of the initial select statement (in my case, from a table variable, in your case from a base table). This particular SELECT statement differs from yours in that I'm transforming the data into something that is more obviously related to the output you require - replacing GLH values with your grade band labels, for example - and (in the second example) also setting up a column for sorting based on the same cases.
Note that the second example brings the data transformation up into the CTE declaration to make the final SELECT statement easier to read. I could also have done this with the first example...