Hello,
I've also done all those things you mentioned: penpals, writing assignments, etc. Everything takes its time, so wait and you will some results.
As far as writing assignments I realised that they didn't pay any attention to my corrections, so they kept on doing the same mistakes. Now they are getting better, because I only underline the mistakes and make some notes, such as SP=spelling, STR=Structure, VT=verb tense, etc. They have to correct them, so they are evaluated twice, one when they first write and the other when they correct their mistakes. It has been successful, you can try that.
Best regards,
Helena