It is something we have discussed here at apps for schools on a few occasions but each time it was felt by the school that the possibility of abuse meant that the pros were outweighed the cons.
There are possible solutions but how practical and workable are these? It would be possible to moderate every comment. This would solve any problem of abuse but this would place a big workload on one or more members of staff.
What about automatic moderation? The system could look out for certain words and block these posts, a good solution but far from full proof.
After this most recent discussion I personally still feel it is not an appropriate medium to provide an open forum, apps are just too closely associated with the schools brand that inappropriate comments from students could have a really negative impact on the impression given to parents.
However I would be interested to hear from any schools that are successfully implementing an open forum of this kind with students in their school. Drop me a line on firstname.lastname@example.org or leave a comment below.