A student says: 'My weight is 54 kg.' What is wrong with this sentence? …

Mass ($\text{kg}$) is how much matter, the same everywhere. Weight ($\text{N}$) is the pull of gravity: $W = mg$. On the Moon $g = 1.6\ \text{m/s}^2$ (more exactly $1.62\ \text{m/s}^2$); on Jupiter $24.8\ \text{m/s}^2$.