“回龙观中学和社区联合演出”用英文怎么说？

The Huilongguan middle school and the community unite the performance

我估计你是要写英文的横幅对吧？那样的话可以这么写：
The United Performance Of Huilongguan Middle School And The Community

若是一句英语句子中的短语的话，也可以这么说，不过需改一下大小写：
The united performance of Huilongguan Middle School and the community .....

the joint performance of Huilongguan middle school and community