The order of your form groups in report and register drop downs are taken directly from the order in your Form Config area. You can see further details on forms here: Form groups - Add, Edit and Delete
If you want to rearrange the order, head to Admin > Config > Core Setup > Forms.
The order shown is determined firstly by whether a form is active, and secondly, the order in which they were created.
If a form group is inactive, it will automatically show greyed out at the bottom of the list - this means there are no students currently in this form.
To rearrange the list, hover your cursor over the form you want to move, and you'll see it become a four pointed arrow. You can then click the form, and drag-and-drop it to change the order.
Once rearranged, this will reflect in how your form list appears across the rest of your MIS.