**Yes** and no, mostly because "tampering" really isn't the right word to use. According to Bill Gates he actually wrote a computerized class scheduler for [Lackside School][1] in which he [included an extra feature][2],

> Of course, a whole new dimension of relevance came when I was asked to
> do a computerized class schedule for the high school.
> 
> It was complex, but ultimately very rewarding. By the time I was done,
> I found that I had no classes at all on Fridays. And even better,
> there was a disproportionate number of interesting girls in all my
> classes.

This is also collaborated in an [interview with the BBC][3] that he did. According to a [Wired time-line on Bill Gates][4], this took place in 1971 so he would have been 15 to 16 years old at the time and was already engaged in [quite a bit of work at the time][5] that he was being paid for as well.

In one sense the [undocumented feature][6] that he added could be considered anything ranging from an [Easter egg][7] to a [backdoor][8] but since he was asked to write the software it wasn't tampering per se even though it is against the [code of ethics][9] of most professional societies.


  [1]: http://en.wikipedia.org/wiki/Lakeside_School
  [2]: http://www.gatesfoundation.org/media-center/speeches/2005/09/bill-gates-lakeside-school
  [3]: http://news.bbc.co.uk/2/hi/business/7464074.stm
  [4]: http://www.wired.com/techbiz/people/magazine/16-06/st_billgates
  [5]: http://americanhistory.si.edu/comphist/gates.htm#tc5
  [6]: http://en.wikipedia.org/wiki/Undocumented_feature
  [7]: http://en.wikipedia.org/wiki/Easter_egg_%28media%29
  [8]: http://en.wikipedia.org/wiki/Backdoor_%28computing%29
  [9]: http://www.acm.org/about/code-of-ethics