I read Tough for a couple of months back, and after I was finished I started with the prequel Koukou Tekkenden Tough. And after i was finished reading that.

I don't understand how the ending of the first story leads up to the second one.
It just feels so wrong, if you get what I am saying. It seems to me as the first series is not completly scanlated. Am I wrong?

Neither series is completely scanlated.

You can easily tell from checking how many total volumes there are, and where the last chapter released is.

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Dropped translations

That was what I was thinking, just gotta read the raws and go with the pictures