This can be shared with the class, and not just the user it was originally for:
Gonna explain this like you are 5 years old.
The phrase "all lives matter" is a reaction phrase, used by triggered conservatives, normally a white person, in RESPONSE to "black lives matter" having been said out loud.
"All Lives Matter" only exists because BLM exists, ALM was created after, and to attack BLM.
BLM was created, because black people are being murdered at an alarming rate by police. Police are also not facing consequences, so legal technicality "police commit homicide on black people". Which is to say black people are killed by police. The argument is, that we need police reform, because Black Lives Matter.
You aren't saying, we need police reform because All Lives Matter. In fact, the ALM crowd are specifically against police reform, even if you personally are for it.
If you really think All Lives Matter then you should also believe the following:
1. Police should have their funding appropriated away from military spending, and to health and human services. (Demilitarizing the police)
2. Contraceptives should be free and readily available, and abortion access should not be impeded. (Family planning)
3. There should be a social safety net, that provides for the welfare of the nation. (Poverty, and homelessness should be eliminated)
These 3 steps would vastly improve social health, by dropping infant mortality rates, reducing teen pregnancy, and preventing disease, petty crime and murders.