I duplicated a layer ("Buildings") in QGIS 3.4.4 and with the Query Builder under Source I selected to just display schools. Subsequently I renamed this layer "Schools" by rightclicking on the layer, choosing Rename Layer.
The new name of the layer appears but once I tick the box to toggle visibility it jumps back to its previous name ("Buildings"). I'm also unable to rename the layer under Source (the 'displayed as' field is greyed out), see screenshot below.
How can I rename this layer permanently?
Edit 21-02-2019: I found that in other projects the layer could be simply renamed or saved with a different name just as was suggested in the answers below. I have not been able to do it with the project I had worked on and suspect a bug of some sort with the imported layers.