In the Gradebook go into Single View, then choose the practice quiz and click 'All' atop the Exclude column. This will exclude this item from the calculations for all students.
I just made a video about this yesterday for a faculty member here, so it is quite fresh for me: https://use.vg/DNCC7d
I just made a video about this yesterday for a faculty member here, so it is quite fresh for me: https://use.vg/DNCC7d