1. The rowStateMap attribute of the managed bean seems to be detached of the datatable: selected items in the rowStateMap do not appear as visually selected in the datatable. When you click the Select First button, the first student is selected (both in rowStateMap and in selectedStudent), but there is no visual feedback from the datatable showing such selection.
2. Every time you click a row in the datatable, the student is selected but it seems the load method of the studentList is getting called again and the table is refreshed, even though the data of the table hasn’t changed, and this causes the selection on the table to be lost. However, in you click the same row a second time, then the selection will work as expected.

see attached example. page acetable.jsf shows without lazy="true" where the selected row is highlighted depending on row selected.
page acelazytable.jsf shows problem with losing highlight of selected row. Also note the button to modify the selected Student by server-side actionListener only works when selectedStudent is in the view (on same page). acelazytable2.jsf contains attribute alwaysExecuteContents="true" which ensures the server-side actionListener is updated to the table, but still client rendering to show selected row is lost.

addition of ace:ajax did not resolve the problem either.