The first graphs showed that people in the age range of students in secondary school had moved from one of the lowest positive test results to the 3rd highest. This was not broken down by age. The graphs you are referring to only referenced the percentage of positive test results in each individual age group. It did not reference how the number of people getting tests taken in that age range had changed.
You need to take both sets of data into account here. The first graphs show that the total number of positive cases has increased among people of all ages since the start of September, but it's increased slightly more among people in the 10-19 age range than among people of other ages. (Unhelpfully, that group also includes most freshers, which muddies the schools data somewhat.) That suggests that more people in that age bracket are currently testing positive compared with people in other age brackets.
The second graphs show that the proportion of tests coming back positive have been relatively stable since the start of September for both primary and secondary age children, while in other age groups the proportion of tests coming back positive has increased. This suggests that more tests are being carried out on school age children than other age groups (which makes sense because every child with a slight cough is more or less forced to get tested or stay home for ten days), which accounts for some of the increase in positive results.
Ultimately neither set of data tells the whole story. You can't just look at the first graphs and say that the infection rate is increasing fastest among teenagers, because teenagers are also being tested more than other age groups, which will lead to more cases getting picked up. On the other hand, you can't look at the stable percentage of tests coming back positive and say that infection rates aren't increasing in children, because the total number of tests is increasing, so a stable percentage of positive tests still means an increase in absolute numbers.
On the whole though, I'd say the data supports the idea that so far teenagers aren't getting infected at a significantly higher rate than the rest of the population. And considering the total lack of social distancing at the school where I work (despite our best efforts), and assuming that's the case in schools across the country, I think that does suggest that school-aged children are less likely to spread the virus - at least amongst themselves - than people of other ages.