I want to list students in order of highest GPA to lowest GPA. How can I do this?

Place a descending sort on the GPA object (Go to Insert > Sorts > Descending). If you have already put another sort on the data (eg Last Name) you will need to reorder the sorts (Go to Format > Sorts)

I want to rank students in order of highest GPA to lowest GPA, but only see the first 25 students with the highest GPA's. How can I do this?

First, you need to add a new variable for the GPA because GPA must be a Measure instead of a Dimension. Go to Data > Variables then Add. Give the variable a name, and make sure it is a Measure. The formula used is =<object> (in this example, the object is the GPA). Place a rank on an object such as ID - go to Format > Ranking, and select a top number of 25, and base the ranking on the new variable you created.