I really do not believe that speaking to a child in a language that is other than English would contribute in him/her being incompetent at school. I mean, come on, you spent 13-14 years in school where everyone speaks in English, you STUDIED the language, you watched cartoons and other TV programs in English, you must have used English language when you were out dealing with people who are not family and you still didn’t learn the language. How is that ever possible? Speaking a language and writing formal write-ups in that language are two different things. There are so very many people in Pakistan who can speak Urdu or their regional language but they can’t read or write.
My parents always spoke to me and my siblings in Urdu. Other than speaking the language, we can read and write it too and I must say that I love the language. It has got certain sweetness and grace to it without having a feeling of snootiness which English may have sometimes ( this is my own opinion entirely). I do see people in family and friends who speak to their kids entirely in English and, sometimes, even when parents can barely speak English themselves and each time I see this happening, I can barely control rolling my eyes.