When a player signs up for HS sports, he/she is committing to the requirements of that sport. If he is to try to also balance part time work, he should make sure that work doesn't interfere with the time slot commitments of his HS sport. A HS coach should make that clear during the tryout/signup process. That player should also communicate with the employer that he is a HS athlete and is not available during those time slots. It really does not work well for the team if one player is allowed regular missing of practice time or games due to other interests - in effect, allowed less of a commitment to the team than his teammates.
That said, once in a while there may be an occasional exception scenario that can be considered and discussed. For example, I have worked with players and allowed once in a while leaving practice early to umpire youth games. Also, during Spring/Easter breaks, everyone's schedule typically gets altered, so we work together as best we can on that. It can be tricky though... once that door is opened, others will look to do the same and/or stretch the guidelines. It is understandable why some coaches will have firm no-exception rules. What is the particular situation you have that you are trying to handle?
I should add... I am also aware that there are schools in some areas where there is very limited enrollment or maybe in a low-income area where it becomes more necessary to accept that some players may have to work and miss practices/games at times. My initial statement is relative to how a typical program is to be run if consistent success and a healthy team/program environment is the goal, IMO.
BTW, welcome to the site!