• Parents: No one likes the bad phone calls or meetings, but they are vital if a student is struggling behaviorally. Set up regular communication. Sometimes the parents are part of the problem (or more than part), but they are still the parents and if you can get them in your corner, you are more likely to succeed. Ask them questions about what has worked before, about what they think might help at school, about how you could better sync up reinforcements at home and at school. And meet with them with the student (and sometimes without). It can give you a whole lot of information and effect change.
• Coaches: If the student has any positive connections with anyone at school, use them. Can they check in with the student? Give you tips? Back you up? If the student doesn’t have any positive connections, is there a club, a sport, or something else the student could do to get them? Because those positive connections are huge for student success.
• Administrators: They get to play the bad guy role and they are paid to have time to delve into what is going on. Even if they say they don’t– it’s their job. At my first school, I had to do the suspensions. That is an administrator job, not mine, and future principals I worked with were appalled. Just beware if you have an administrator who likes to make things your fault– if you do, cross you ts and dot your is first. 
• Guidance counselors: Many are overwhelmed but they are good if you think there is a crisis at home or elsewhere that is behind what you are seeing. They are good are figuring out the whys– but not typically good at implementing behavior plans.
• School psychologists: Many are stuck doing nothing but paperwork, but their training is much broader than that. They are trained to understand behaviors and to design interventions for them– and will do it often if pushed. Their job is not to work with just students with special needs either– they are part of a school’s intervention team, which means you can call on them about any student you are worried about. It’s more complicated at a big high school where things are compartmentalized a lot– but at smaller schools, they will, most often, respond and be actually helpful.
• Speech and language pathologists: Their  expertise is not behavior but they understand social language and communication. Sometimes students need social stories or ways to interpret and understand a situation more effectively. If so, SLPs can be helpful– and if you think there is a communication need, even with a general education student, it is good to loop them in.
• Occupational therapists: Their expertise is self-regulation (and fine motor skills). That means they know about programs like Zones of Regulation and have materials and ideas to share. Unlike the other folks though, these often are only looped in after students have IEPs. If your student doesn’t, I recommend bringing a bribe and a smile to see what you can get.
• District people: Big districts have entire behavior support departments that will work with students with and without IEPs. Smaller districts often don’t, but there still might be someone down in central office who can give ideas and suggestions. The district is on the hook for the money if a student with an IEP has to go to a more restrictive placement for behavior so if the student has an IEP and is not doing well, you might suddenly find someone will to help out if you say that you really don’t want the student to have to be out placed.
• Outside doctors and therapists: A release is a beautiful document. To talk with any outside agencies, you need a signed release from the parent. If you can get one though, it can be huge. If there is a therapist, you can let them know what you are seeing. If there is a doctor overseeing medication, you can also share what you are seeing. A lot of times students go for appointments and say everything is great– and parents often like to have someone share something about what they are seeing.

There are a lot of intervention ideas and resources on this page but they might not be right for your student. Start Googling! And check out these toolboxes. This one, from a PBIS group, has 17 pages of behavioral interventions. This page from Intervention Central, has a bunch more.