It's difficult to pinpoint what percentage of the world has heard the gospel. There's no way to track who has and who has not heard the gospel. Someone in your English class may have never truly heard the gospel (though they have access), but someone living in a Muslim country where Christians have virutally no access may have heard the gospel.  

When missiologists (people who study how to spread the gospel throughout the Earth) they usually look at people groups rather than individual people. Here are some statistics they have found.

• According to the Joshua Project (as of February 16) there are 16,598 people groups in the world. 7,165 of these are “unreached” (fewer than 2% evangelical).
• On the other hand, The Southern Baptist International Missions Board estimates 11,310 people groups, of which 6,405 are unreached and 3,100 are “unengaged” (no evangelical mission effort to reach them is underway).
  
  

Either way you look at it, there is a huge percentage of unreached people groups. At first this can look very discouraging. However, consider this statistic.

• There are 700 million evangelical Christians in the world — 225,000 for every unengaged people group

There are enough evangelical Christians in the world that we have the potential to take the gospel to every people group within our life time.  

HERE is the article where I got my statistics from.