Is everyone in your class able to hear a quiet sound equally well?

No

Explanation:

Loudness describes how people perceive sound (see loudness). ... If people could hear equally well at all frequencies, the contour lines would be flat because the same measured sound intensity would be perceived to be equally loud regardless of the sound frequency. In fact, people do not hear as well at low frequencies.

0.500% of 0.7988 is

0.005 • 0.7988 = 0.003994

The absolute difference between Kelvin's measurement has to be within this threshold. The absolute difference between what he found and what's worth an A is

|0.7925 - 0.7988| = 0.0063

but this is larger than 0.003994. So Kelvin did not make an "A".