Baseline Option tab
Set this to Fixed
(turn on preview at the bottom to see the result before pressing ok)
Insert a value in the Fixed Basline to get the text flush where you need it.
One more question.
Is there a way to apply this to every text box used, so that the text always top aligns to the text box?
Set that Frame as an Object Style (Window>Styles>Object Style)
(make sure you tick the option to apply the style to the currently selected frame)
Then do a Find/Change
Go to the Object Tab
Select the option to find just Text Frames
And then select the object style you would like to apply to it.
To alter the Text Frame (for everyone in the document) you should just have to edit the Object Style.