my DD3 used to be like this. in fact, at parents evening they were stunned to hear her speak to me, as they had never heard to speak to anyone (even me at drop-off) before.
she is now 10, and much better than before, altho still has her moments. the teachers often struggle to hear her when she does speak, altho she will converse happily with her friends now, which she also never did when small.
only problem we are left with now is that often when someone talks to her, she just stares back at them, which is very unerving, even for me! I asked her recently why she did it, and she seemed genuinely shocked as according to her, she responds.
problem is, I think the thinks the words, and expects us to just know.......we are not telepaths tho.
one bit of advice, try not to make an issues of it, as it gets harder to get over then. my DD3 was yr2 before she started to talk without being spoken to first, and probably yr4 before she voluntarily approached the teacher/put her hand up in class. she is yr5 now, and like I say, much much better, but still not what I called 'relaxed' (IYGWIM).