One thing that might help you out is speaking so they understand you easily. i dont really know what kids study in HS, but if you limit yourself to just things they have studied.
look at the grammar page
http://jhsenglipediaproject.com/grammar.aspxIt will show you everything they have studied in JHS, so stick with those grammatical structures, and maybe they will understand you better. maybe.
relative pronouns tend to be pretty difficult for most people i come across, so use them carefully.
for example "My biggest problem as a teacher when i had just started was not knowing what the students know."
that being said, grammatically the above sentence doesnt have anything they dont study in JHS, but listening to it would probably be pretty difficult. sice it uses 6 things they learn at very different times.
knowing where to take pauses helps too. Id take small breaks if i spoke this sentences based on the structure like...
My biggest problem/ as a teacher/ when i had just started /was not knowing /what the students know
anyways, figure out what they know and use it. be repetetive with your words at least at first.