Hm...I too would be very cautious about adding an SSRI to handle rages. In my personal experience, rages are associated with
1) Hypomanic or manic symptoms
2) Mixed states (energy of mania plus despair of depression)
3) STRESS
It's pretty unusual for a kid to be doing well at home and having melt downs at school. As I understand it, what's more typical is for a kid to hold it together at school (due to social consequences with-peers) and to melt down at home. In which case I would be looking very closely at possible triggers in the school setting.
If this hasn't been examined closely I would go there first before adding an SSRI, especially if he's in a regular classroom with no aide or significant interventions like chunks of time in an RSP setting or other very small group setting.
In my experience, both with myself and with my son, medications can help but they are not a magic bullet. Many other factors like routines, triggers (sensory issues or too many changes too fast), stress (esp. in the form of expectations that are perceived as unreasonable or impossible to meet), criticism (or language that could be interpreted as critical) - these can make a vulnerable child lose it despite all the medications in the world.
Identifying and then reducing or appropriately managing issues like these can be almost as important as getting the right medication combo in our experience. The medications help your child's brain calm down (for lack of a better analogy) but medications don't get rid of the basic tendency toward instability and don't eliminate things like poor sensory integration.
Plus, if your child has been unstable for a while the pathways in his brain that lead to melt downs/rages are much more easily triggered than ones that help maintain his stability. That's why avoiding these episodes through environmental management is, in my humble opinion, one of the most important things you can do for your child right now.
After a sustained period without big meltdowns, my son is much more able to handle stresses that previously would have led to volcanic eruptions. so I have put a very big priority on avoiding, directly or indirectly, triggers that I know are likely to lead to rages and actively managing a mini-melt down so as to avoid a major one. Figuring all this out has taken a couple years so don't get discouraged. Just stay focused on your goal and on the incremental improvements you see.
Based on our experiences, your child is likely to benefit at least as much from a sustained period without rages (created through artful management of his schedule, environment and interactions with others) as from medication tweaks - once he has been on a mood stabilizer for a while and you have seen significant improvements overall.
At the same time you are really pushing the school about changes there, if it were me, I would be wondering about the following medication tweaks before adding an SSRI (but I am not a psychiatrist and a lot depends on your child's presenting sx). These are pretty general ideas that may be completely wrong but may give you a starting place for a more in-depth conversation with your psychiatrist:
1) change in mood stabilizer - has he tried Lithium? Or has the psychiatrist suggested adding a small dose of Lithium to the mix? I would suggest adding Lamictal but Lamictal and depakote don't mix well and it would be best to reduce or wean him from the depakote before adding Lamictal. Otherwise it has to be added very slowly.
2) Has your child had his depakote levels checked recently? Also is he being closely monitored for liver toxicity and metabolic effects? If these things aren't happening, I would personally be looking for a new psychiatrist. These should be checked often, especially in a child so young.
3) Try a different atypical anti-psychotic before adding an SSRI
Finally, I would be also looking at supplementation of his diet with Omega-3s, Vit. D3, and Carnitine. You can have his Vit. D level checked if you want before supplementing this and I would definitely talk to his peds about how much Vit D3 (best form) to give him. Recent discoveries have shown that Vit. D works more like a hormone in our bodies than a vitamin and very low levels are associated with many different health problems.
I suggest carnitine (an amino acid) because depakote strips this amino acid from the body. Some people are poor synthesizers of carnitine and, when on depakote, cannot maintain an adequate amount in their body without supplementation. My psychiatrist had me on this when I was on depakote. She said that it couldn't hurt (apparently there's no great danger of overdosing) and might help. Or you can feed him large amounts of red meat including liver.
And there are actually some studies that show that Omega-3's have a positive effect on the stability of people with bipolar. Here's a link with more info on this.
http://www.psycheducation.org/depression/medications/Omega-3.htm
Again, this is something to discuss with psychiatrist and peds. The link above discusses possible risks/side effects and is a good place to start.
Best wishes