We present spectroscopic observations obtained at the {it Large Binocular Telescope} in the field of the cluster XLSSJ0223-0436 at $z=1.22$. We confirm 12 spheroids cluster members and determine stellar velocity dispersion for 7 of them. We combine these data with those in the literature for clusters RXJ0848+4453 at $z=1.27$ (8 galaxies) and XMMJ2235-2557 at $z=1.39$ (7 galaxies) to determine the Fundamental Plane of cluster spheroids. We find that the FP at $zsim1.3$ is offset and { rotated ($sim3sigma$)} with respect to the local FP. The offset corresponds to a mean evolution $Delta$rm{log}(M$_{dyn}$/L$_B$)=(-0.5$pm$0.1)$z$. High-redshift galaxies follow a steeper mass-dependent M$_{dyn}$/L$_B$-M$_{dyn}$ relation than local ones. Assuming $Delta$ log$(M_{dyn}/L_B)$=$Delta$ log$(M^*/L_B)$, higher-mass galaxies (log(M$_{dyn}$/M$_odot$)$geq$11.5) have a higher-formation redshift ($z_fgeq$6.5) than lower-mass ones ($z_fleq$2 for log(M$_{dyn}$/M$_odot$$leq$10)), with a median $z_fsimeq2.5$ for the whole sample. Also, galaxies with higher stellar mass density host stellar populations formed earlier than those in lower density galaxies. At fixed IMF, M$_{dyn}$/M$^*$ varies systematically with mass and mass density. It follows that the evolution of the stellar populations (M$^*/L_B$) accounts for the observed evolution of M$_{dyn}/L_B$ for M$_{dyn}$$>10^{11}$ M$_odot$ galaxies, while accounts for $sim$85% of the evolution at M$_{dyn}$$<10^{11}$ M$_odot$. We find no evidence in favour of structural evolution of individual galaxies, while we find evidences that spheroids later added to the population account for the observed discrepancy at masses $<10^{11}$ M$_odot$. [Abridged]
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