The 15- 19s isn't uni students is it though , if we match against your MSOAs, they ares till testing positive in their droves. The fall in known cases is surely that 16 year olds, many 17 year olds and all 18 year olds are not in school at the moment. That may be keeping them cosied away and actually safer from the virus, or they may not be testing, or both!
To give my school as an obvious example -as we only have years 9 and above, all our cases are in Year 9 (so 10-14 year old graph) , a few in year 10 and year 12 but they are on study leave atm. So the vast majority of what we are dealing with is year 9 aka the 10-14 age group. We have very little oversight of anyone in year 11 -13 at the moment.
I don't know ,therefore, whether that is a genuine drop, or an unknown quantity.