Hi :) is "Ben" necessary in this phrase?
It is totally optional :)
Would okulda just mean 'at school'?
When do we use "dayim" and "deyim"
We use deyim or teyim when the word ends with front vowels such as marketteyim, evdeyim, dersteyim, otobüsteyim and dayım or tayım with the words that end with back vowels like okuldayım, sokaktayım, yoldayım etc.
the major rule of Turkish is the vowel harmony, if you haven't read about it so far, please do http://www.turkishlanguage.co.uk/vh1.htm