My website was on 7th page before yesterday.. As you can see it's a sub domain of my main website But today when i checked my website on google search i cannot find my sub domain. I can see my main domain on 10th page.. But the strange thing is that whenever my main domain appears in search results, sub domain gets disappear. Why this happening everytime? Keyword is "Web Based School management software"

Hmm, let say you are using that subdomain to add some conntent, bot (that index your page) get to your page see that it is updated and try to reindex it once again. Checking links to that page, how content is displayed, do you have real content, and many many other things.
So google bots are visiting your page apx. every 2-3 days and do this reindexing.
And based on your website did you change something or not, and how good, big, valid is your change you will get better results.
Of course there are many more things like keywords, phrases, how do you fill your h1,h2 content, etc...
So major reason for this doesn't exists, that is the way it goes, what you can do is to try to do always best prcatise so your website will be faster and better ranked on google.
One more note is that for other search engines like BING you can't see your post at all.
I have a blog you can try to search in google "is2 hql" and you will have it on first link that is Hibernate Query langauge but if you do exactly that on BING he will not even find you, just because blogger belongs to google and they don't want to rank it at all...
Hope I didn't make more confusion.

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