enum.php

<?php
Schema::create('asistencia', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->unsignedBigInteger('id_alumno');
    $table->dateTime("fecha_y_hora");
    $table->enum("estado", ["a", "r", "f"]);// <-- Aquí el enum
    $table->foreign("id_alumno")
        ->references("id")
        ->on("alumnos")
        ->onDelete("cascade")
        ->onUpdate("cascade");
});