How does one best translate 老在 into english I couldn't  find any translation in my dictionaries. Though. In context it seems to mean 'always'?! 
这孩子老在班上捣蛋。
Is this correct.

Cheers
老在 is not a word. It's an adverb followed by a preposition.

老 means always or constantly.

在 is just a preposition, here it would be 'on' to go with 班上(the class)

So it's: This kid always create troubles on the class.