Hi Babybean!
When I did this unit I had a word with the SENCo as she is the person who mostly gets in contact with outside agencies. I think the main point here for most schools regarding policies is that contact with outside agencies must be done through the correct channels. for example, if you suspected a child in the class you work in had a particular condition, you wouldn't get in contact with the agency yourself. You'd probably express your concerns to the class teacher (chances are they'd have noticed something too) and it would be up to the class teacher to talk to the SENCo who would in turn speak to the appropriate agency.
If you are working with a child who already has contact with an outside agency e.g. speech and language therapy, you may be required to talk to them about how that child is progressing. You would then need to know the school's guidelines regarding confidentiality so that you would know how much you could tell the sp & lang therapist when they came in. You would be able to tell them how that child was getting on regarding their sp & lang targets but would not be able to discuss any of their personal details.
I hope that gets you started anyway!
Good luck!
SN