Start high priority backup job in front of other queued jobs

if the resources are already in use and available for lower priority jobs, then those lower priority jobs will start in preference to your high priority jobs if resources are not available for the latter (the main resource here would tend to be tape drive(s)).

Again, to concur with Bob, the only was around this would be to suspend those lower priority jobs that are taking up the resource(s) that are required for your high priority job. This should also include any queued jobs that are liable to utilise those same resources - if you suspend only the active jobs then, as the media is still loaded, the queued jobs will then become active.