In this day an age at the high school level, coaches and athletes should not have each other's cell phone numbers or be involved on any social media that allows private messaging (for our sport, Strava should be fine because there is no private messaging, but the coach should clear this with their AD, school district, and/or school board first).
Many districts approve the use of something like the Remind app for quick and easy communication between coaches and athletes that keeps personal cell numbers hidden, and all messages are easily viewable by the district. This goes a long way towards keeping things appropriate because there is so much transparency.